You'll have to decide whether you want to make a really good assassin, a really good thief, or a really good PvE character.
I love playing my PvP thief and he does occasionally assassinate bots, but he could not stand toe-to-toe with a good PvPer or a high-level mob.
The problem is, a good thief needs to have - at minimum - stealing, snooping, hiding, and stealth. Thus, the remainder of your template is limited to three, possibly four skills. A character with three or four skills geared toward PvPing will be at a sharp disadvantage against a character with seven or eight skills geared toward PvPing.
If I were you, I would create three different characters: a thief, a PvPer, and a PvMer. This separation of templates will make you more effective and much less frustrated.