Frank is somewhat nicer overall than his comic counterpart, and becomes more violent towards the end of the film. This is consistent with the behaviour of real-life killers who gradually escalate their crimes.
After leaving John Saint to die, Frank leaves his Punisher logo bulletproof vest (which was shredded by bullets in the earlier gunfight) behind with him. This seems to be a hint that they were originally planning to have John Saint become Jigsaw (a plot point which the Punisher video game used), due to Jigsaw being known for wearing a copy of Frank's outfit to mock him. This is further indicated by the fact John Saint is the guy who actually kills Frank's wife and son, and has quite a lot of presence in the film despite not having a direct role in the plot.
