Yes. Some people will come up and pk you for "no reason". However, the reason may be that they are evil people who enjoy killing. It may be that they want to find out if you have any good armor.

In general, you should be safe if you stay hidden (except from duergars, assassins and other thieves).

Don't worry when you get killed, because it will happen (a lot).

About getting in your guild, you need to pay your dues. This is (from what I recall of my time as a thief) 50 gold. ie. 50,000 copper. You might want to just go to an alternative guild in the meantime.

Alternatively, and I am surprised more people don't try to do this, you could get a high level character to sponsor you. Personally, I'd be happy to pay for a few thieves to join the guild, on condition that I could call in favors from them in later life.