Stop Co-GMing.
Clearly, there are at least two (and, it sounds like there may be three) competing ideas for what The Campaign should be. Since you've tried (and failed) to convince Tim that his "tweaks" and loot are making it impossible for you to run your sessions in the same campaign, stop sharing the campaign: his style and yours are simply incompatible (note: neither is necessarily "wrong" or "bad", just "different").
Depending on the desires of the rest of the group, there are two major options:
- you can bow out gracefully ("I'm sorry, this isn't for me, but have fun!")
- your group can rotate campaign worlds (and, perhaps, systems) instead of just GMs
- on a regular cycle (weekly/monthly/...)
- between "chapters"
- between "campaigns"
I'm in a group that's rotated successfully for years, even with slightly different players (one person worked a rotating shift, so was only available "2 on, 2 off"; that helps); I've also had friends gracefully bow out without causing problems re-joining later when a new game/campaign was starting. I've also had friends who had a long-running "God-mode" campaign interspersed with "Krazy Kobold Adventureses", which acted as a palette cleanser between "chapters" (in that particular case, they were explicitly in the same world, but the Kobolds heard about fantastical things that were happening "way over there" and never directly affected them).
Pretty much all rules dictating deity worship are gone in 4e, replaced with DM discretion and recommended threats of story-based consequences for unpopular actions or beliefs within a subculture.
Additionally, there is nothing in the rules preventing a character from worshipping multiple deities, cherry picking which parts of each that appeal to them. The PHB openly states, “Most people revere more than one deity” (PHB pg 20), but most people are under the impression that, because the online tools only allow you to select a single Deity, that players can only worship one. This is point-blank untrue, and furthermore, wouldn't make much sense within a provably polytheistic society.
So your character could worship Sune because she embodies the ideals of Beauty and Trickery, and another evil deity of your choosing that embodies torment and madness. The two would combine nicely in your concept.
Another twist I've used before successfully is a character who believes that she worships one deity, but who is in fact interacting with another entity entirely.
Perhaps the most well-known iteration of this kind of story arc is Pelor the Burning Hate, but it doesn't have to be a universe-wide deception by the deity, it could just as easily be a case of an individual or single temple being fooled by a fraudulent impersonation of their intended deity.
In this case, such a character could honestly believe that they are worshipping Sune, but their prayers are being intercepted and answered by Cyric, the Prince of Lies, who is attempting to gain enough influence to break free of the prison that Sune helped build for him.
His alignment could have aided in the slow corruption of your PC, and the illusions afforded him by the trickery domain would be neatly augmented by the madness and strife slowly creeping into your PC's demeanor.
It's up to your DM how to adjudicate things such as feats, paragon paths, or epic destinies which require one to worship Sune, but thankfully there's only one of each, and none of them are stellar picks.
Best Answer
In order to know how to deal with this player, you first need to figure out why he always wants to play an evil character.
Possibility 1: He may have lingering biases from previous editions of D&D. You tagged your question 4e, so I'm assuming that's what you're playing. Previous editions of D&D had much stricter alignment systems, and often playing a "good" character meant dealing with a lot of tricky rules (see: 3.x paladin). If the player feels like the only way he can avoid these rules "gotchas" is with an evil alignment, then you should explain to him that 4e is different and he can safely play an Unaligned character.
Possibility 2: He uses D&D as an outlet to do evil things, perhaps for catharsis or stress relief, or simply a desire to be evil that he can't indulge in real life. If this is the case, then you need to decide if you are willing to accomodate him using your games as an outlet to be evil. If you're not willing, or the rest of the group doesn't enjoy it, then you need to have a private discussion with the player and explain that. You can say, "I run games where the PCs work together, and a single evil character in an otherwise good or unaligned group doesn't fit with that. You're welcome to game with us if you play a good or unaligned character, but I won't be accepting evil PCs into this game."
If you are willing, however, then look for ways to give him motivations which may be evil, but still align with those of the group. There are a couple of ways you can do this: either out in the open, where the other PCs know he's evil but work with him anyway; or kept secret from the other PCs.
As an example of an out-in-the-open motivation, perhaps the player's a would-be warlord, but a lich is encroaching on his desired territory, so he wants to kill it. The other PCs want to take down the lich simply because it's doing evil things. The evil player would benefit from allying with the other PCs regardless of alignment to take down the lich, while the PCs would gain a powerful ally, even if they don't agree with his motives. The other PCs may even hope that the lich and the evil PC kill each other, thus taking out two major evils at once (assuming your group is mature enough for that kind of PVP plot).
As an example of a hidden motive, perhaps the group is seeking a powerful relic buried deep in a dragon's den. The PCs want to use its power to save the world, but the evil PC wants to use it to overthrow the king. The evil PC may hide his alignment and pretend to have the same motive as the other PCs; or he can be openly evil but claim that he, too, wants to save the world "because I'm one of the idiots who lives in it!"
TL;DR: Assuming the player wants to play evil characters as an outlet for his own evil urges, you need to look for ways to link his (evil) motivations to the party's non-evil ones.