People do indeed kill for the kick of it. But professional robbers stick to robbing, sometimes going as far as to not use real weapons in case they get caught. But there are all kind of offenders, some with mental health probelms or some panicking who end up killing even if they did not intend to do that initially.
Most robbers are not "professional robbers".
And incidentally, in case you didn't know, to classify as a robber you need to either use, or threaten, with violence.
The average burglar is not a robber.
Professional robbers aren't really a thing. It's more a lifestyle, and it tends to be quite short. Many people who would consider robbing others would kill for 50k, because most robbers and thieves are not rational profit maximizers, to say the least. 50k is the kind of money that makes normal people start getting ideas, let alone criminals.