Is Snape evil or not?

Interview scripts for Book 1 to 4
Interview scripts for Book 5

See Half-Blood Prince updates for further details

From the interview scripts I've been able to find on the Internet, Rowling left me wondering if she intends Snape to end up good or bad after all.  If you recall my saying this in regard of Stephan Fry's interview at Albert Royal Hall, Rowling kept telling us to keep an eye on Snape. Affirmation which highly disturbed me since I just cannot fathom the fact that Rowling still wants us to think Snape is a bad guy.  I've been discussing this with my friend, Afictionado.  Here are my conclusions:

If Snape turns out evil...

• it would explain why Snape never overcame his hatred towards Harry, because in the end, he would want him dead or to use him
• Albus would become an instrument for Snape
• it would confirm why he favors Slytherins and associates with Lucius Malfoy
• it would also explain why Dumbledore never gave up the DADA position to Snape. After all, he did appoint charlatans like Lockhart, and to a worst point yet, Umbridge who was chosen by default since nobody else could be found to take the position.  We did learn that Dumbledore was in charge of assigning teachers, not the Ministry unless there was no other options. And still, Albus would not appoint Severus even though he was appointed as Lupin's supply-teacher when he felt indisposed.  Is Dumbledore so unsure of Snape's capacity to withhold his attraction for the Dark Arts?  But at the same time, he sends him out on spying errands does he not? Snape is therefore trustable enough  spy but not to teach DADA. Like my friend, Afictionado, said, there is more to it than Snape potentially being evil. We also know Snape is knowledgeable in the Dark Arts from Book5.

On the other hand...

• it would indeed be a turn of events unworthy of Rowling's plot quality.  I personally would be quite deceived that, yet again, another dark, lonesome, misunderstood character be the prey of others. Just as the Phantom of the Opera where, in the end, the Phantom turns out good but has nobody to understand and support him except after his death.
• saving Harry Potter again and again would have been for nothing at all. And what about Snape always protecting the kids!  Why would he give a damn?
• If Dumbledore trusts Snape enough to teach Potions which could be deadly or even Dark, why does he refuse to let Snape teach DADA?  For the students's protection?  I don't think so, unless he would not be teaching at all. I don't think it has to do with Snape being evil or not.  
• Why bother to teach the way he does?  I mean, he is mean and sadistic, but he teaches and expects perfection from his students.  Someone who would only be biding his time would not really care now would they?  Out of professional concern? I don't think so!  We know Lucius is very bad, he does his "job" well because it will bring him connections, wealth, respect and trust. That's why he is acting differently with the likes of Fudge.  However, he has his own agenda at all times while doing so. He is preparing.  While on the other hand, Snape is preparing for what?  Teaching! Snape puts his efforts into teaching, not planning unless that is a totally unrevealed part of his life.  

Hence, my answer is still NO. Snape is not evil.  He has a bad character, can be a bastard at times, but that's who he is.  He does what he must.