As a Marijuana connoisseur for over 12 years, which includes growing and many other aspects I'll give my take, even though they've been answered already a million times in this thread..
1. Can weed make you hallucinate?
Sure can. If you eat it, it's worse because you can't gauge how much you need and when it'll kick in. You can eat one cookie, not feel anything and eat a second, only to have the first one kick in right after you've eaten the second. By that time, it's too late. When you smoke, you literally have a few minutes of a gap between when you puff puff pass and the effects are taking place. Marijuana can make you hallucinate but as has been said, not to the effect that LSD and other hard drugs give. The most I would say it does is alter your perceptions. For example, the other night I was walking around Niagara Falls blasted drunk and high off my ***, everything I looked at with the light and the nighttime started to make the trees, water and grass, basically the environment, look like it was straight out of Crysis, but I didn't see little asians popping up with guns.
2. Can weed worsen mental illness/trigger schizophrenia?
Yes. Marijuana has been linked to being a possible cause of Schizophrenia in people later in life. I think a study concluded that people who smoked before their brains could develop properly (I think it was anyone under 18-21) increased their chances of schizophrenia 4x compared to someone that never smoked. However, take into account just how many hippies there are out there still kicking it without any issues and consider how many of them have schizophrenia.
3. When you are under the influence of weed, is it difficult to act like you're sober?
It's the same as alcohol. It depends greatly on the person and how much they've smoked, etc. A lot of people I know can handle their weed, and don't act any different when they are sober compared to being high. On the other hand, my girlfriend can't act 100% normal, or at least thinks she can't. For the most part, Marijuana doesn't impair your judgement to that degree, and you're still able to completely function. If you're an idiot to begin with, you're probably going to be a bigger idiot when high and unfortunately, there are a lot of idiots out there.
4. Is weed very addictive?
No. Marijuana has been proven time and time again to have zero physical withdrawal symptoms. The only things you may notice are slight irritability for the first couple of days or difficulty sleeping for a few nights, and these are only if you are a constant smoker. If you're someone that smokes a joint every weekend or at parties, when you aren't high you probably won't even think about it.
5. Does weed affect your judgement?
This ties in completely with part 3 somewhat, but yes it can to some degree. Again this is greatly dependent on the person and mscbuck gave the best answer I've seen thus far to this one. I can safely say that when I am high I have NEVER gotten the idea that I could fly, and I'm completely aware of what is right and wrong. You aren't out of your mind at all.
6. Is quitting the use of weed associated with withdrawal symptoms?
Only if you're a person that smokes a lot. See my answer to #4.
7. If you're feeling anxious or low to begin with, will weed just strengthen those feelings?
I wouldn't recommend using Marijuana to get away from those feelings first. However, this is a difficult question because it also depends on the person. For the most part, Marijuana does not make you anxious or low at all, in fact it does quite the opposite. However, if you smoke too much over your limit of it, you can have a bad trip. All you have to do is learn your limit and you'll avoid this easily.
8. Is there a lot of bad/contaminated weed out there?
This is a very misleading question because the real true answer is No, however it has happened in the past. Logically, it does not make any sense for a drug dealer to lace Marijuana with any harder drug because Marijuana is so much cheaper compared tot he hard drugs. They wouldn't want to waste their possible revenue in the hope of getting someone hooked. Not to mention that most street dealers (usually your friends) aren't so high class that they can afford to even do so. It has been known to happen int he past just like the mixing of any drugs by people. There are some easy signs to tell if your weed has been laced with another drug such as tasting a bud, and if your tongue goes numb it's laced with crack usually. Or just look at it, crack a bud open and see if the inside differs with the outside, etc. As for "contaminated" weed I assume you mean just weed that's gone bad or something, which happens a lot more often than laced Marijuana, but still even this happens very rarely. If you smoke moldy weed, you will get INCREDIBLY sick. It's a very bad situation to be in. However moldy weed is also easy to spot and even smell, so you'd have to be extremely dumb and not notice either of the two while cutting the weed up and rolling it. In all my years of smoking I have never once had moldy or laced weed.