. >>> tommy lynn sells is a 46-year-old man on death row for a single murder. but by his own admission, he killed so many people over the years he lost count. that level of violent evil makes sells brain fascinating. and as part of tomorrow night's "nightline" primetime special, martin bashir sat down with this unrepentant monster. >> reporter: they were all innocent children. a son. a brother. lives with limitless possibilities ahead of them. but what these boys would grow up to become, brutal killers. could their brains have provided a road map to the sickness that would consume them? >> the first time i killed somebody, it was such a rush. >> reporter: tommy lynn semis was once a little boy growing up in st. louis. but around the age of 14, he says he became addicted to killing. how many people have you killed? >> lord, i don't know. i -- i don't know. >> reporter: ten? >> yeah. >> reporter: 20? >> probably. >> reporter: 30? >> it's up there. >> reporter: 50? >> see -- i'm not billy the kid making notches on my holster, so -- i know it's been a lot. >> psych

. >> reporter: tommy lynn sells was once a boy growing up in st. louis.ound the age of 14, he says he became addicted to killing. how many people have you killed? >> lord, i don't know. >> 20? >> probably. >> 30? >> it's up there. >> 50? >> you see, i'm not keying, making notches on my holster. >> reporter: sells is on death row for one murder. but he's linked to at least 17 more. the drifter's vagrant lifestyle helped him elude police for nearly 15 years. his victims turned up from coast to coast. >> i am hatred. when you look at me, you look at hate. >> reporter: his methods for killing were as random as the people he targeted. he raped many before cutting their throats or beating them. stabbing others and strangling some. dr. adrian rain has studied the brains of people scientifically determined to be psychopaths. >> one of the key differences that we find in the brains of psychopaths is that there's a structural impediment to part of the brain called the medulla. this is a part of the brain very much involved in the generation of emotion. >> i like to