Even if you're example is a bit extreme, I am pretty sure that a 4 years old has not the maturity to understand the far-reaching consequences of such dire actions. Understand that it is bad yes, understand that it is definitive no.

This is also why depending on the jurisdiction and their age, minor are judged according to special laws. And I am not sure a place exists where a 4 years old would be put on trial.

The lack of unconditional love is such a weird concept to me.

At least in my case, it's not revocable; I couldn't stop loving them even if I wanted to. And yes, that includes your hypothetical extreme example. My children will still be my beloved children regardless of their actions.

So would you start hating the killer or start hating yourself for not teaching good vs bad.

Plus - considering the other comment - the concept of permanency is - at least in my very short experience - a difficult concept for a toddler.

I've thought about that, having multiple kids and having lost one. My prediction is that I still would love the murderous child. It's rather surprising how strong my emotions are to my little girls for no conscious reason.