>>95841598 (OP)I'm going to assume we're talking about D&D, since the concept of a "party" doesn't go far from that. So, good in the early game. Coming across rival delvers who don't appreciate you touching "their" loot is a classic conflict. Highly distracting, and unnecessarily dangerous in the later games. D&D player characters scale wildly out of line with monsters by design, so hostile entities designed as players will invariably be monumental threats, especially if you throw them at the party with them fully charged and ready to nova, while the party has already exhausted any level of their resources.
Ultimately, you need a good reason for the evil party to have gotten so far and become so powerful and yet still insistent on being bastards who can't just get along. Very difficult balancing act, and all too easy to make them nonsensical and outright stupid elements that still blow you up for no good reason.