Cruel to each other, or cruel to their victims?

In either case the answer varies - pirates were people, some appear to have been vindictive, bordering on psychotic, others were perhaps quite reasonable people (apart from the whole robbing from people thing).

There are plenty of examples of pirates taking a ship and leaving the crew and captain unharmed - sometimes even paying for the goods they took (at a price dictated by the pirates, obviously) or exchanging good with them. On the other hand, there are also plenty of examples of pirates treating their victims with great cruelty. Victims were sometimes killed by the pirates, and torture and violence were not entirely uncommon.

For the most part, this violence was not senseless, but was used to make an example, get revenge on an individual, or, most commonly, to force the crew into revealing where valuables had been hidden. For example, when Edward North and his crew were captured by Charles Vane in 1718 they

barbarously treated this Deponent with all his Company by beating them and using other cruelties particularly to one, who they bound hands and Feet and ty’d (upon his Back) down to the Bowspritt with Matches to his eyes burning and a Pistol loaded (as he supposes) with the Muzzle into his mouth, thereby to oblige him to Confess what money was on board. (The Deposition of Edward North, 22 May 1718. CO 37/10, f.37)

Sometimes pirates got violent through anger, as in the case of Richard Hawkins, who was captured twice by Francis Spriggs' pirate crew. The second time, he had little or nothing left to steal, which rather annoyed the pirates:

I was surrounded by fifteen Men with keen Cutlashes in their Hands, who all made at me, and soon laid me on the Deck, some giving me the Edge, others favour’d me with severe Blows with the Flat. Here I should certainly have expir’d, had it not been for Mr Burridge, who flew among the thickest of them, and begg’d earnestly to save my Life, adding that he would never beg for another. Whereupon, he being so well belov’d, they ceas’d beating me. (The British Journal, 8 August, 1724)

Sometimes however, pirates were fairly cruel just for the fun of it. Hawkins again:

After this they wanted a little more Diversion, for Mischief is their sole Delight: I was sent for down to the Cabbin to Supper; what should be provided for me but a Dish of Candles, which I was forc’d to eat, they having a Pistol at my Head, and a naked Sword to my Breast, whilst others beat me with Swords call’d Tucks. After that I had eat to their Satisfaction, I was buffeted and thump’d forwards to the Bag, among the rest of the Prisoners, who had much the same Fare with myself. Then they consulted for more Diversion, which was to sweat me: It was agreed on and all Preparations made thereto. The Manner of a Sweat is thus: Between Decks they stick Candles round the Mizen-Mast, and about twenty five Men surround it with Points of Swords, Penknives, Compasses, Forks, etc. in each of their Hands: Culprit enters the Circle; the Violin plays a merry Jig; and he must run for about ten Minutes, while each Man runs his instrument into his Posteriors. In I was called, in order to enter the Ring; but Mr Burridge advance’d among them, with the Captain, Mr Cole, the Surgeon, and a Prisoner on board, who had formerly been Gunner of her, these, with one Joseph Cooper, did intreat earnestly for me, alledging, That I never did any Man any ill; that I had done them no Injustice; and they had already ruin’d me; wherefore they begg’d I might be voted clear of any farther Punishment;

This account reveals one of the very interesting thing about pirates' violence towards their victims. If the crew who served under a particular captain vouched for his good behaviour, that he had never been a cruel master, there was a good chance the pirates would let him off, but if they reported that he'd been a cruel master himself then the pirates might make their punishment that much more severe in response. William Snelgrave, captured by Jeremiah Cocklyn's pirates went from being under a death sentence to being a more-or-less honoured guest when his crew spoke up on his behalf.

In terms of being cruel to one another, it's quite complex. Some pirate ships had systems of justice clearly emulating legitimate courts on land, with a jury of twelve men and a prosecutor. Jacob du Bucquoy described the process on John Taylor's ship thus:

When a crime has been committed by a member of the band, the quarter-master proceeds against him in the name of the law in front of a jury of a dozen members, of whom half are chosen by the accused. The latter, having presented his defence, retires, and the jurors pronounce judgement which the quarter-master executes with fairness and impartiality.

What really differentiates these pirate courts from legitimate courts is that pirates didn't have any real mechanism for extending mercy in the same way the legitimate courts did. On land it was not uncommon for judges to direct a jury against the evidence to acquit a defendant, or for a jury to do it on their own decision, and even once a felon was convicted the judge had scope to be lenient in sentencing AND recommend a pardon from the King. Amongst pirates, though, once a man was brought before the jury there was little chance of him escaping punishment unless he happened to receive arbitrary support from some popular member of the crew, such as Harry Glasby, who was sentenced to be shot for infringing the rules of Bartholomew Roberts' company, but 'escap’d it only by one of the leading Pyrates being his Friend, and bullying the rest'

In terms of actual punishments, pirates do seem to have been much less restrained than their legitimate counterparts. I've not done a statistical analysis, but the impression given by the sources is that the death sentence was handed out proportionally much more often by pirates than by legitimate courts, and without any mechanism for mercy it's almost certain that a great percentage of death sentences handed out by pirates were actually carried than on land. In terms of corporal punishments, it's very easy to see that pirates were MUCH less restrained than, say, the Royal Navy or the East India company. In the Royal Navy a full court martial could order floggings with a very large number of lashes, but courts martial were not common and were reserved for the most serious offences. On a day to day basis the largest number of lashes a Naval captain could order on his own authority was twelve, and although the East India Company never had a ruling on it, twelve also appears to have been the de facto limit for their captains. By contrast, in all of the surviving sets of pirate articles in which flogging is mentioned, the standard number of lashes is 39, even for relatively minor offences. That number was often exceeded as, for example, in the case of Benjamin Jeffrys, a member of Roberts' crew, whom witnesses described at his trial as having once received six lashes from every man in the company, which consisted of 296 white men and 70 black men (differentiated because it's not at all clear that black men were considered full members of the company so may not have been included). Thomas Jones, also in Roberts' crew, claimed to have once received 2 lashes from each member of the company, but later served on a jury that sentenced two men to 500 lashes each. (Captain Charles Johnson, A General History of the Robberies and Murders of the Most Notorious Pyrates, London, 1724; The Information of Edward Evans, 10/10/1723. HCA 1/18, f. 37)

So, short answer: yes, pirates probably were more cruel and violent than their legitimate counterparts.