it is fundamentally inhumane to kill someone that knows it’s coming (mental torture)

That killing serial killers causes them harm isn’t a particularly compelling point, since we disagree over whether harming them is, in fact, good.

risk of executing an innocent

This is a good point and one I would explore further. However, it leaves open exceptions where the evidence is overwhelming.

it is hypocritical to kill someone for killing

Killing isn’t always bad. Killing innocent creatures is bad. Killing serial killers is tantamount to putting down rabid animals.