Best friends tend to have your best interests at heart, or at least think they do (even if they're kilometers off), even though sometimes they'll doubt themselves. Complete bastards have their own convenience at heart, although they can be quite convincing that really, what they want you to do will actually be good for you, and in your own best interests.
Both best friends and complete bastards have the unique ability to so thoroughly say or do the wrong thing so as to make you feel like complete shit. It happens. Complete bastards may not care so much, but may make with the elaborate apologies and promise to never do it again. (Sometimes, they'll do that very thing again near-immediately.) Best friends will usually feel like hell if they screw up like that, and may make elaborate apologies (or not so elaborate, depending on the friendship) and promise to never do that again. (They'll really try hard not to, but they may slip, and it's a near certainty that they'll find some new, unique, delightful, and completely different way to screw up just as bad or even worse later, sometimes near-immediately.)
Both complete bastards and best friends will critique you on the things you do wrong or could be doing better. Complete bastards tend to make you wind up feeling lousy about yourself, especially if they tell you that they're telling you this for your own good. Best friends sometimes try to use tact, and sometimes fail miserably, but are determined not to allow you to make a certain mistake. Either party may tell you loudly, in a public place, that what you're wearing makes you look like a zebra that got run over in a school crossing, but be assured that the complete bastard will tell you from malice aforethought, and your best friend will tell you that so that you won't wear that gawdawful thing ever again.
When you screw up, as you inevitably will, the complete bastard tends to make you pay for it five to ten times over. They'll do their best to make sure you regret that so much you'll never do it, or anything like it, again. The best friend will call you on it, or at least they'd better, and somehow, things keep going on. You feel lousy because you made them feel lousy, not because they made sure you felt how lousy you made them feel again and again until you agreed to major concessions; that's the province of the complete bastard.
When the complete bastard comes out and says that their goal in life is to make you miserable, you have an open and honest enemy, and that's fairly easy to deal with. When you have an undoubted best friend who does everything right, it's clear that all is as it should be and you will keep this friend forever if possible. Those kinds of bastards and friends are easy to identify.
The most difficult complete bastards to deal with are the ones who use social rules as weapons, leave you ready to kill yourself, them, or both with no onlooker the wiser for what they just did, and leave someone less slick looking like an idiot if they call the complete bastard out for what they're really doing. The most difficult best friends have no social skills and a penchant for saying the worst possible things at the worst possible time, and they usually find a way of letting you know that you mean the world to them, even if they can't articulate it particularly well. You just know that you can't stand the bastard, even if it makes you feel guilty, and for whatever reason, with your best friend, even though they may be irritating as hell, you feel good around them and wouldn't lose them for the world.