First: Many a thiefrogue has pilfered from the party. This is nothing new or unusual. In AD&D the richest guy in the party was the thief (well... mine anyways).
Nothing prevents any character pilfering from the party. I'd expect my character to get Perception rolls to notice that, however, and my character just might be one of those Barbarians who thinks the proper penalty for thievery is loss of a limb. Or maybe he's a head in the clouds wizard or cleric who cares nothing for worldly goods, so why worry about it?
I've never run a "thief". Thieves are petty villains. I've run a few adventurers and rogues, though. If they wanted to be thieves, they'd probably stay in the city where risk is lower and marks are plentiful.
Second: Robbing the party doesn't mean rooting through their packs. I usually quick grabbed stuff off the bodies or handed the nabbed something small from a chest. I was also pretty good and distracting the other players;
If you're in the back casting spells, you're not at the front near the bodies and the chests, are you? Although I'd expect them to look at the lockpick to deal with the chest in any case.
Third: Evil players do not need to slaughter. Not sure where you get that notion from. (Its wrong.) Even an evil guy can concede that he needs to use whomever he can to get what he wants. This means backing up the party as usual.
Not sure where that comes from. You're the one who mentioned "mass killing". Most PC's, from plunderer to Paladin, kill their enemies, and I expect most characters would consider someone I caught pilfering from our PC group who then directs a spell my way to be an enemy.
Currently a friend of mine is running Souls for Smuggler's Shiv. He told me that their rogue stole the loot from the Janeviere (the boat which wrecked them there). However the thief has no idea what the potions are and has forgotten where he hid them. In game terms, their expedition is totally screwed (so many things are poisonous), but its just part of the story.
A bottle of poison mixed with the potions could be harmful as well. But hey, just because my character caused a TPK doesn't mean it wasn't a great idea for a PC who'll help the party, right?
I'm less concerned about getting found out than you seem to be. The thief is often at the back... if the attention is forward, spells are safe and easy to cast. In the mean time he can fill the role of thief. Even if I'm spotted, they still can't put the exact god in the picture. (Thief covered in Calistra fetishes... nothing new there...)
Yes and no. Many spells have visible effects. Of course, you can stick to less obvious spells.
At some point, the other characters may start wondering why the rogue never enters melee combat, but a bow and some decent range combat feats can put paid to that question pretty easily. I find characters move around in combat quite a bit, so facing the "back" of the party isn't that uncommon. And those spellcasters tend to like hanging back as well. Maybe a step back to the Thief/Rogue is between me and the enemy...
To the original issue, I'd be inclined to rule that the holy symbol can be disguised/used without prominent presentation. The nature of the deity is such that forcing his followers to loudly proclaim their allegiances seems off, somehow
All this aside I am humming and hawing over my options. I have to concede that being more neutral is more flexible, and more acceptable if I get found out. In my head I'm also going in circles about trying to meet thiefly goals, while butting up against the feat tax of trying to create the character I want.
Trying to be two different characters will definitely create some issues in that regard.
