Because AFO was born evil, and shigaraki was manipulated into it by AFO. It was strongly emphasised that the influence the villains get is what determined wether they turned into a villain or hero, and because Deku started in a similar situation he wants to save Tenko a chance, and “kill” Tomura.
To be perfectly honest, being born evil is also such bullshit excuse they used to have a reason to kill him instead of helping him. You could argue that deciding to be a villain is worse than being born a villain, because it involves a choice to be bad.
Welp, we will see how it will progress, but yeah, it's pretty obvious how it will go :"/
I hate that constant narrative in shounen mangas that you have to save everyone or you aren't "good". Bro, this fucking guy is evil. he is no different to AFO, but AFO can be killed because he is bad. Why Shigaraki isn't bad enough to be killed two?
Typical japanese shounen nonsense, eh, but at the end shigaraki will still die and Deku is still gonna save the day by being better than everyone. Like it wasn't done a million times by now