How is that different from believing there is nothing after death? Why wouldn't that effect a person and their ?
People do things for a lot of reasons, and the claimed reason is seldom the true answer. If you want to believe that believing the possibility of salvation after committing a horrible crime, makes someone a bad person, that's as good a reason as any.
Do you think consequences of actions impact how a person acts? You should, because a century of science confirms it (not to mention every moment of everyday experience).
However, it is not the actual consequence that matter, but the beliefs about the consequences that are the proximal impact on actions. Real world consequences only have an indirect influence on future behavior via impacting beliefs about consequences, which are what determine actions. IF those beliefs deviate from objective consequences, then objective consequences no longer influence actions.
All of the "many reasons" for which people do things share the common property of being believed consequences of those actions. And almost all of those reasons are beliefs that the action will improve the actors own well being (whether in the form of short term pleasure or satisfying long term desires). Afterlife beliefs completely alter the calculus of what those consequences appear to be, as does the belief that you'll be loved by a God even if every human on earth despises you for the action. And if you believe that this God has its own criteria for rewards that are not contingent on the objective harm you cause others (as most theists believe), then you are more likely to act in ways that have no regard for the objective harm you cause others.
It is rather simple. If the other kids on the playground punish you for picking on the gay kid, you are less likely to do it. But if your daddy only gives you love and rewards if you pick on the gay kid, then you are likely to ignore the consequences from the other kids and pick on the gay kid. God beliefs operate no differently than actual rewards and punishments by one's earthly parent, except they are often much stronger effects on behavior because the consequences are believed to be more severe and lasting.
Likewise, if you know an act will be unforgivable to all humans and they will want to kill you or punish you for the rest of your existence, then you are less likely to do it. But you also believe in a God whose will forgive and reward you anyway, and that his opinion is the only one that really counts, then you become far more likely to commit that act than you would without that God belief.