Jim's done a lot of really bad things. The worst? Well, it's subjective as you say, but I think probably becoming a murderer at 12/13, and killing an 11 year old boy for laughing at him has got to be up there. He killed a lot of people as an adult, but idk, showing himself to be that vengeful that early - and also being unable to deal rationally with other people - is pretty bad.

Though I'm a bit torn, because blowing up a block of flats and killing twelve people just because an old lady described the sound of his voice...also pretty bad.

How much does he regret any of it? Not a single bit.