I agree with KM and Dragonbait. There was nothing here that was out of line.
#1. Relevant hooks. As the DM, imo (and I am always the DM/GM), it is your responsability to sit down with the players and find out their characters' backgrounds and motivations.
#2-3. Derailing the adventure. If the other pc's can't convince the hold out, the player sits out the session. Or, perhaps, the DM throws in a few short cut scenes with the remaining PC and then supplies a situation to bring the PC back into the adventure. Worst case scenario, the player retires the character.
Ex1. I was playing a monk in a friends campaign. In his campaign world, there was an order of monks, who were very paladin like. Be an exemplar of good. Protect the helpless and down trodden. My character was from that order.
At one point, the party came to town in another country. People were on the gallows set to be executed. When member of the crowd were questioned, the party learned that these people had stolen bread, fruit, etc.
The rest of the party wanted to ignore the situation for the time being until they learned more about the country we were in. The party tried convincing my character to turn a blind eye and threatened to disavow my character if he got involved . However, it would not have been in character for the monk to stay idly by and let these people die for such a petty crime- the GM agreed (and the players agreed). I was prepared to retire my character.
We played out the rescue. Then, the DM focused on the group. Rather than have me retire the character, the GM threw occasionally threw out random news of a vigilante throughout the session. Occassionally he did a quick cut scene to have the monk rescue people from criminals (including one that was stage in attempt to capture the monk). However, most of the play focus was on the other PCs.
He brought my character back in by having him catching the attention of a local group of freedom fighters. They received word that the other PCs had attracted to much attention asking questions and were going to be ambushed.
One of the individuals that the monk had rescued alerted him and he arrived to disrupt the ambush and the party was back together.
Ex 2. I brought in a replacement character. The PCs were in another nation far from their homelands. My replacement character was from the local desert nation and worked with the PCs for the adventure. However, when the adventure concluded there was no reason for my PC to continue with the others to their homeland as he had family matters to deal with at home. So, I simply retired the character.
#4. A character going berserk out of grief-not a problem. Let the party deal with him. Maybe the party restrains the character or casts a spell if they think his behavior is harmful to them.
In my most recent supers campaign, one of the PCs shot a restrained captive in the head. Two characters turned in disbelief . Another player heard the shot and got word over the comlink as to what happened.
Quickly the two characters nearby used thier power to restrain him.
The offending player thought I would let his actions slide, because he was a PC. He was wrong and the others promptly turned his character to the authorities to stand trial. (I may or may not use a mind control scenario after talking to the offending player as the victim was a minion. I doubt it, but the possibility is there).