i guess he can be, but maybe the past avatars would make an end to his way of life, or even his life. if they take the second option, the avatar will reincarnate again, and hopefully be a better version then his last one. i dunno if the previous avatars are strong enough to defeat the current avatar though... maybe he cant, but then again, they would be able to force him into the avatar state and take him over (cause in the avatar state, the avatar is in direct influence and connection with his previous lifes). i dont think its likely that the avatar gets ''evil'' though... why would he? i mean, he has complete temples for his own, and people worship him... seriously, if the avatar wants something, he'll get it anyway, he wont have to get evil by killing people for that.

in this case its a bit different though, cause people lost their respect for the avatar cause he hasnt done his job in protecting the world.

If an Avatar was born into the Fire Nation in the middle of their war then they'd use the Avatar as their weapon...

If you think a little harder you will remember that the job of the avatar is to balance everything, in the recent avatar world the fire nation is controling everything, thats why aang is helping the oder nations, but his job makes him do stay as a neutral element, and not an evil or good guy...

thats only a theory I'm not saying that the avatar couldn't be evil...

I think it plays into a cycle. Do firebenders see themselves as evil? They might think their cause is to make the world better or something. Therefore if the avatar was brought up in teh fire nation, what he thought was right may be seen as wrong, hence evil. So really the avatar can't be good or evil, its just teh way peopel think of him as. A fire nation child might see him as evil, hurting their way of life, while a water tribe clihd might see him as good, improving their lives.

So in conclusion, no persons good or bad, its just a perception from one person to the next. so really there is no answer.?? glad i could help