No, most murders are not cold, calculating events where the perpetrator has no remorse.
Brothers are not the same nature (DNA), unless they are identical twins. It’s a very complex subject but the various traits of a psychopath are probably 50:50 genes and environment, but in terms of brain development it’s difficult or maybe impossible to separate the two. The easiest way to understand genetics is that it’s a recipe for how an organism develops, with a lot of variation depending on environment. The basic wiring problem where emotions like empathy are completely lacking appears to be entirely genetic, but the personality traits could be better or worse depending on environment.