I don't really think this is valid reasoning tbh. Governments can kill people at a whim, but frequently do not because they would rather they die over time through conditions such as prisons. There are other factors they consider apart from simply wanting him dead. They don't need to have killed his directly. It could simply be the result of mental and physical health issues due to his imprisonment. Life expectancy in prisons is markedly lower for a reason.

I've seen takes that he was killed by the West to blame Putin, but I haven't really seen any actual hard evidence for this

Western governments want Assange dead. So by that logic he'd be dead long before now. He's not, but I'm not about to conclude that the US gov doesn't want Assange in an anonymous ditch. There are plenty of revolutionaries being let to rot in US prisons from the previous decades. It's just killing them in slow motion.

At the end of the day we don't have objective info to allow us to conclude one way or another as to exactly why he's dead, and both the West and Russia are obviously deeply biased sources.