did the Viking mentality of 'Victory or Death' give them the edge?

This is very much a modern 'Viking mentality'. If we had to define a contemporary 9th Century 'Viking mentality', it might be "there's rather a lot of them, let's go somewhere else." The reason the Danes are so effective in Britain in the 850s - 870s is because they try as hard as possible to avoid fighting anything like 'fair' fights with the English. After all, when the English do meet the Danes in large-scale pitched battle in the period, they tend to win; a Danish force which raids the South East in 851 defeats a Mercian force sent against it, but is subsequently defeated by a West Saxon army with 'such a slaughter as had not been seen.' Another Danish army which attacks Winchester in 860 is subsequently interdicted and destroyed by a West Saxon army in pitched battle.

Avoiding what we might call a 'fair fight' as much as possible is key to successful "Viking" strategy; The Danish army which invades Northumbria in the late 860s does so at a time of civil war in the kingdom, and is able to capitalise on neither side burying the hatchet to unite against the invader until far too late, for example. The central pillar of this strategy, however, is in rapid manoeuvre warfare and diversionary raiding, and the successful implementation of this can perhaps best be witnessed in accounts of 871, as Alfred ascended to the throne of Wessex.

In 871, Asser (Bishop of Sherborne and biographer of Alfred of Wessex) complained that Wessex was being 'bled white' of manpower, not by Danish victories in pitched battle, but rather through the constant need to dispatch forces to form local garrisons and try and prevent myriad raiding by peripheral forces of the Great Heathen Army trying to tie down the English and leave them vulnerable. Myriad raiding was designed to force English forces either to remain localised to protect their own lands, or be continually counter-marching in order to suppress raiding, resulting in the English armies going into battle either heavily under strength, or physically exhausted, and thus avoiding a 'fair' fight us much as possible for the Danes. Indeed, the West Saxon army faired reasonably well against the Danish army in 871 even when faced with diversionary raids and Danish manouvre warfare: elements of it won a victory at Englefield; the larger army was defeated at Reading, but clearly not particularly heavy since it was then able to inflict heavy casualties on the Danes in a bloody stalemate at Ashdown. A fortnight later the Danes won a victory at Basing, but then suffered heavy losses at Meretun two months later. The strength of the wider Danish strategy can be seen in the final pitched battle of 871 at Wilton: Alfred's army was significantly outnumbered by the Danes, having had to dispatch a great part of its strength to what Asser calls "countless small expeditions" and, although it was able to rout the centre of the Danish army and acheive victory of a sort, it was unable to pursue for fear of being surrounded and overwhelmed, and instead chose to withdraw in good order. This strategy gave the Danes a decided tactical edge until Alfred's defensive reforms of the mid-870s, which considerably changed the strategic picture in England.

While there will be always more to be said, you can check the following posts by /u/BRIStoneman on the relevant topic:

• Were most vikings also famers? If so why were they so much better warriors than the saxons
• How were the english able to defeat the vikings?

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Neither do I think that the Danes had their superior tactics in England in the late 9th century, as I explained before from a bit different point of view in: Early Viking attacks on England failed as often as they succeeded. But after a few decades, they seemed to be far more effective. Did viking warriors improve or did viking leaders develop better tactics? Why did the Vikings become deadlier over time?