Many think in terms of good versus evil,but evil also fights evil and good, good. Powerdull greedy forces out there could crave the renown and power they would gain from taking them down.

Sucessfull evil can make other evil greedy for their power and wealth. Like 2 organized crime groups, when one flourished they gather the attention of the other, or several street gangs duking it out for power or territory.

Sure some evil forces may seek them out and ally themselves with them,but others might see the players as growing threats and decide to take action against them. If they get stuck being attacked by both fronts things could get fun.