In a multiplayer game, if you Mindslaver an opponent, can you make them attack another opponent?

Mindslaver's effect gives you control of target player's next turn. During that turn, you make all choices and decisions the controlled player is allowed to make — including declaring attackers and choosing which players (or planeswalkers) those attackers attack. This is explicitly supported by CR 718.2 and 718.4, which govern player-controlling effects.

In a multiplayer game, a player may attack any opponent or any planeswalker controlled by an opponent unless a restriction prevents it (CR 506.2). Since you are making all decisions for the controlled player during their turn, you can direct their creatures to attack any other opponent at the table — even the one you consider least threatening, or the one who is your ally.

The controlled player has no say in any decisions during their turn. You choose which creatures attack, whom they attack, what spells to cast, what abilities to activate, and so on (CR 718.2). The controlled player can't override or veto your choices.

Example: It's a four-player game with players A (you), B, C, and D. You cast Mindslaver targeting player B and sacrifice it. On player B's turn, you direct all of B's creatures to attack player C instead of you, then use B's hand to cast spells that benefit your own board state.

One important note: you are still bound by the game's rules when controlling the player. You cannot make them take illegal game actions (CR 718.4). For instance, you cannot have them attack with a creature that has summoning sickness or is tapped. But choosing targets among legal opponents is entirely within your discretion.