It was bad that Engine Joe was a killer because Tom and Huck have heard of him before and they both watch him kill Doctor Robinson. Killing someone is a big crime because you kill a person that did nothing to you if they did you don’t go and kill them you can reason with them. Killing an innocent is not good, and your family because they would miss you if you are in jail.
If Tom and and Huck does not tell someone what happen about Engine Joe in the graveyard he could find Tom and Huck and kill them both.