I know that the rifles were very inaccurate, that's why they used "line infantry". But how effective were the rifles in killing people?
They were very inaccurate at landing hits, but very effective at making people go out of fighting if the hit actually lands.
In 18th century, despite some small differences, majority of European (and subsequently - American) infantry was armed with smooth-barrel flintlock muskets that functioned more or less the same regardless of the weapon's model. Aside from usual concerns such as bad training and absence of proper aiming mechanism we have in modern rifles, these firearms had rather poor quality by today's standards: around 10-15% of shooting attempts would usually end in misfires even in dry weather [1]. Even light rain would heavily increase number of misfires and heavy downpour would make shooting completely impossible as the gunpowder would become wet while being loaded. Moreover, due to bad quality of gunpowder and imperfect weaponmaking techniques musket characteristics could degrade over prolonged periods of time and even during the single battle: not completely burnt out powder may clutter the barrel and make it harder to reload or even damage it, the barrel itself may become very hot and this will require cooling it down, soldiers will also get tired and shoot with decreased efficiency.
Outside of technical difficulties, old firearms' relative inefficiency would also be hampered by very complex reloading pattern that would make it impossible to have good rate of fire. Excellently trained soldiers in perfect conditions could make as many as five shots per minute, but average passable level only expected a common soldier to shoot thrice [2]. Even worse, continuous firefight would hamper these results even more since it was very difficult to stop people from needlessly wasting ammo once they decided their life was in danger. Even the most effectively drilled infantry would eventually lose self-control in the heat of battle and start hastily reload and shoot without proper aiming. French military theorist Jacques Antoine Hippolyte de Guibert argued that only the first salvo made by line infantry was truly effective because muskets had been carefully loaded before the battle, but once the fight commences, people would lose their cool and conduct reloading and shooting procedures carelessly.
Average shooting distance would be around 100 and 200 paces with several armies and commanders demanding soldiers to start firing only when there were as few as 30 paces between them and the enemy. French training manuals stated that infantry fire at around 135 meters can be devastating yet shooting at distances bigger than 230 meters is a waste of ammunition [3]. By utilizing extremely precise weapon and in perfect conditions it was possible to achieve landing a hit at one kilometer. In the end, there usually was no reason to shoot further than 450-500 paces away because due to the loss of energy a bullet couldn't inflict big damage on the target. Due to this, a lot of thought was given to properly organize and form infantry ranks in order to get proper results from shooting: in late 18th century many statutes would prescribe leaving around 0.5-0.6 meters of free space per footman, i. e. packing them as tight as possible [4].
Bullets used in muskets were quite big by today's standards, up to 20 mm in diameter. What they lacked in precision they would make up in inflicting grievous wounds upon the target, and it was common for soldiers to become outright casualties even if they had only been hit in legs and arms as it was impossible to continue the fight. But in any way, line infantry firefights could go on for hours on end, and during this time armies usually utilized cavalry and artillery to quickly rout enemy forces.
TL; DR: not very effective in total, but very effective when rare lucky guys could hit the enemy.
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More insight into this matter is always welcome, so if anyone wishes to contribute a post of their own, feel free to do so! While we wait for that, OP, you may be interested in this previous thread on just the same matter, with a post each from u/PartyMoses and u/dandan_noodles.