Are you using a 'poisonous bite' weapon?

I'm not 100% sure whether or not stat loss is bad for NPCs, but yeah, most of the Thug skillset is for PVP.

Level 12: push
Level 14: kidney shot
Level 17: gouge
Level 21: weapon butt blackjack
Level 26: parting block
Level 28: payment of the coward
Level 29: sucker hit
Level 30: gut shot
Level 31: death strike
Level 35: third attack
Level 36: strike on the abandoned
Level 38: earclap
Level 42: prey on the weak

...Mostly PVP. Third attack is helpful in PVE as well. Weapon Butt Blackjack can be used to knock out NPCs for whatever reason... Parting block can keep flee prone NPCs (there arent very many) from running away.

Most of your non PK skillset comes from the core thiefy skills. Hide (Most NPCs cant see you) Detect hidden (good for exploring) peek (see what's in a target's inventory when you look at them) lockpick.

Thieves tend to be very gear-reliant, as they need every advantage they can get to shore up their squishyness and general frailty.

As a player getting started in the game, I suggest you use the unofficial wiki (http://diku.qhcf.net/cgi-bin/wiki.pl Make an account in the QHCF.net forums to access it) for maps, gear advice, and play guides. Practice lore, and look at everything (Items, NPCs, signs, hidden descriptions in rooms, etc) to collect observation experience which will help give you % in lore and access to new useful skills.

Keep a list of gear to collect when you've died and lost your set. Remember areas that tend to have lots of stuff on the ground (you can determine item value with Lore) and try to sell them at assorted shops. This gives you commerce experience, and access to new useful skills.

Running around, exploring the game will also give you explore experience which gives you extra movement points, which really add up after a while.

The biggest piece of advice I can give you, is keep your chin up even if things turn crappy. Play your character till agedeath or con death, and you'll learn a hell of a lot, making the next character even more awesome.