I'm ocd though, and I wanted them to stay...I play neutral good though, and didn't act too nice so I don't think my acts forced them to leave.......I was in the middle of a wilderness area and they said they couldn't wait any longer, as if there was a time limit on getting them to their destination or something.
Otherwise, i'm fine with NPC party members wanting to leave IF they want to leave because of my actions....but if they're timed to leave afetr a certain period in the game code then I don't like that sort of thing.
The time limit existed for them, not for you.
You're thinking of these characters as if they exist to serve you. This isn't your run-of-the-mill RPG where party members are robots created to carry out your every command. This is Baldur's Gate. The characters have reasons for doing (or not doing) everything, including turning on you in battle or insisting on a particular course of action. That's what the "role-playing" aspect of the game is all about.
Regarding your loss of the pair, it is probably just as well. Montaron is Neutral Evil and Xzar is Chaotic Evil, which basically means they are selfish, self-centered, and not above doing whatever it takes to get to their goal. They have a specific mission and a time-limit to get there, and you weren't keeping up with their schedule. To be even more specific...
"In Baldur's Gate, Montaron can be found along with Xzar on the road near the place where Gorion was killed. He's a member of the Zhentarim dispatched along with Xzar to find out who's to blame for the iron crisis, since their organization actually has nothing to do with it in spite of rumors to the contrary. He hates working with Xzar and generally seems to think that the best thing that could be done with everyone besides himself would be to kill them, something which he openly enjoys. He and his counterpart get along particularly badly with the two Harpers, Khalid and Jaheira, with whom they may eventually come to blows."
If you know anything about the Zhentarim, you know that they aren't patient with hirelings who fail to fulfill their contracts. Given the choice between continuing on with their mission and being dead, I'd say those two picked the better option.
If you really want to enjoy BG and BG2, you will need to pay attention to the alignment and personal interests of those in your party. The other option is to "dumb the game down" by using a hack to turn your party members into automatons, but that sort of defeats the purpose of playing an RPG.