You are also right from other pov.
How to answer by being cruel against cruelty? How would we be better than that criminal? By being merciful. Yes, death for the proved serial or mass murderers is the best punishment
My judgement is influenced by the latest events here. I am thinking at this lately very much with disapointment about the presence in society of these monsters who in fact are humans like me.
Btw have you seen the NatGeo documentaries about WW2? The conclusion was brutal, the war criminals were normal humans, some of us are just normally monsters.