This is because we can be of two minds about these things. You can have a personal response to heinous acts, but still think the government ought to be better.
If some guy murders the murderer of their kid, I can absolutely 100% understand why, and I could even admit that I might do the same in their position. But I still think that as a society we should not lower ourselves to this standard and I will always be against the death penalty (especially because the system will never be perfect and I will never think it’s worth killing even one innocent person by accident).
It’s why vigilante justice is so easily understood, but it’s still something we, as a society, shouldn’t accept.
Emotional reactions can cloud our minds to these things. But I absolutely agree with you. This is horrendous and barbarous. I can still somewhat understand the “he deserves it for what he did”-response, but I’m absolutely against this on a deeper level.