"I am having fun this way and I'm not a liability to the party so there is no problem."
From what I can tell you're going with a character who's more about support spells rather than outright damage spells. Which, given your good charisma might actually be a very good choice. But there's two sides to playing D&D: the combat and the roleplaying. So you need to ask yourself this:
- What do I do in combat? Your build is not one of setting everything on fire, but you still have to do something to not be a liability to the party. If you have considered this, and thought about what you do if the monsters no-sell all your Suggestions and Charms, you should be fine. This is important for all characters by the way: what happens if your main thing does nothing against the enemy.
- What do I do out of combat? As a character more geared towards roleplaying you should have a leg up in non-combat encounters. Make sure to point this out to the party and ask them how they deal with non-combat situations. Do they have spells that can do this? Do they have skills and high charisma to smooth-talk their way around? If not, well there you have it. If they do, selling your character is a bit more difficult but still managable.
Just remind them of the fact that if you're having fun and not a load to the party, they don't have a leg to stand on when critiquing your choice of character.
First, you're off the hook for "My Guy" syndrome - this is clearly a conflict the GM orchestrated. "My Guy" happens when a player decides to use his character to justify derailing the plot, but telling you that your lost love is working with the enemy means that this is the plot. You're being presented with a genuine challenge that hinges on your buy-in of this conflict, and I'm willing to bet this was exactly how the GM hoped things would play out.
Second, you need to start looking for the shades of grey between "slaughter the cultists" and "join the Abominations". She's worth a lot more as an ally than as a corpse, and having her working as a double-agent or informant will give your group a better chance of stopping the cult for good. Taking her out prematurely removes your best source of information, and you might never get the access you need from anyone else.
Furthermore, you said she wants out of the Cult, which means Redemption is a possibility. If the Inquisitor can't find mercy, compassion, or hope enough to treat her as anything other than an enemy (in which case, they should probably be Lawful Neutral) convince them that she deserves something other than summary execution. Take her into custody, let her face justice and shine the light of truth on the whole cult. As soon as we move from "shoot her on sight" to "demand her surrender", then we buy time to redeem her.
You should also look closely at that "really only there to help a friend" angle. If you can rescue this friend, or otherwise get them out of trouble, then the cult loses its leverage over her. At that point her motivations will become much more clear, and she should be able to become a reliable ally.
Of course, betrayal is always a possibility, but you're usually better off trusting and getting burned in an exciting plot twist rather than shutting down the story early to avoid the risk. Optimism is more fun.
Best Answer
A badly chosen approach to a situation can guarantee narrative failure in spite of mechanical success. I'm fairly confident that's what happened here.
From the info you've given, the situation was that your character had a cultist at his mercy. Just describing them as a cultist suggests a primarily fanatic devotion to a cause greater than themselves, and in some cases a willingness to die furthering said cause or even an eagerness to be martyred for it. Your GM confirmed that this was largely the case. If the cultist wants to protect cult secrets by taking them to the grave more than they want to live as a betrayer, offering to kill them if they don't comply is literally giving them what they really want. That's not intimidation, no matter what the skill check die shows.
The GM describing it as successful Intimidation wasn't wrong, if your roll beat the DC, but it definitely sent you the wrong message. In fact, there's an argument to be made that your GM shouldn't have let you roll at all. Letting you make the Intimidation check in the first place, if they knew your chosen approach would make the task impossible, gives you the illusion that narrative success was still possible if you rolled well enough and sets you up for disappointment if the check Succeeds. What they could have done instead was make the failure of the approach more visible: either through in game descriptions such as "At the mention of death, he looks suddenly at you like a kid in a candy store." or outright tell you "You just threatened a death seeker... with death. He laughs and dares you to get on with it. You'll need a different leverage on him if you want him to spill the beans."