What if it IS what my character would do?

[Glorthindel]

You're missing the point.

the point is that in this scenario, "kill slavers whenever we encounter them" has been a permitted behavior for the entire campaign so far.

It is only when the PCs are enslaved by a corrupt Government after being framed by a crime that killing the slavers—in this case,t eh people actively trying to enslave the PC—becomes disruptive.

But that's my point - it's only a problem because the PC set the extreme requirement of "they must die or I must die". Sure, it was fine when they were just running into groups that the PC could take out, but there is always a bigger fish, and when the PC ran into that bigger fish, their ethos came apart, because they were stuck with one and only one strategy "I must kill every one, or die trying". His one-dimensional character trait dictated he couldn't attempt to bribe or barter with factions of the slavers to bring down the other slavers, that he couldn't bide his time and wait for an opportunity to break free, all because "slaver: must die immediately" was his only setting. And that's unrealistic. If his life goal was to end slavery, there is much more efficient (and likely more successful) means than just attempting to kill every single slaver on sight, immediatly and regardless of his own safety.

To blame the DM for presenting a situation that the PC's narrow inflexible 'personality trait' couldn't handle is not a problem of the GM; it's his goddamn job to present situations that the party have got to plan and strategise to overcome. It is absolutely the players fault that he is treating his characters personality like a line of computer code (see slaver - attack immediately). It is not the DM's obligation to go "well, better never put a single slaver in the game who can't be immediately killed with no repercussion", its the player's obligation to portray a reasonable and rational character.

I'm going to explain myself one more time. If I have to explain it again, I am going to assume that the breakdown in communication is not my fault and begin acting accordingly....

It's not the other players whose fun is being ruined here.

The GM made a choice that disrupted the game. The other players, including the one who chose to have his player act consistently, made no choices, they just continued playing the game the way they'd been playing it the entire time. This ruined what the GM had planned, and ruined the GM's fun.

Looks like we posted simultaneously, so I will edit to address this; its possible we are discussing different ways the game was "disrupted" - if the DM was expecting the party to switch personality and suddenly by ok with slavers (and the player continued to oppose them 'in character'), and that not doing so "disrupted his game", then yes, he's a moron and not reading his own room. Where I am coming from is if the player is expecting every enemy to be instantly killable with no repercussions, and then the DM finally presented someone who couldn't be beaten in that manner, then it is the players fault for presenting a personality trait so one-dimensional that he isn't able to adopt an alternative strategy, and feels "forced" to disrupt the game by enacting the "or die trying" half of his personality.