Purposeful killing

I was just taking with my husband. And we were talking about the girl that just drove into the pedestrians killing one person injuring many more. And we were debating what the punishment should be. I am very pro the death penalty I feel like if you purposely killed someone with 100 percent proof then you should have the same done to you. He debated me saying that sometimes cases come up of people being accused wrongly innocent people die. But just out of curiosity does anybody disagree that if you know with 100% certainty the killer and that they killed like with this girl for example caught on video and all should you get the death penalty? And if you think no: what do you think about lawyers always trying to blame it on psychiatric issues to get the murderers sentence reduced? Also what do you think the punishment should be if not the death penalty for an 100% obvious and proven murder?