Percy is no more a death eater than I am.
Percy, let me go on record as saying, is a proverbial stick in the mud. Among his family, he is the rebel choosing a misguided loyalty to law, order and the ministry over what he deep down knows is the truth and that is that loyalty to Harry and Dumbledore must be protected at all costs.
I could go on for pages about Percy and his relationship with his siblings and the fact that all the teasing gets at the hands of his brothers contributed to his rule-following nature, but I'll just stick to the "Is he evil" topic. Debates rage in the fan community about him. But I have always thought he was good, just young and foolish. He's ambitious, yes, but evil? Hardly.
Percy has spent six books following rules to the letter and for his efforts he's been reward -- or so he thought. In fact, all of Percy's promotions have been a sham so that the Ministry can keep tabs on the Order and Harry Potter. Percy, unlike is father, is sadly easily manipulated because he desired to get ahead always gets in his way. His father as much as said that to him and it caused a rift in the family that may never heal. The ministry is not on the side of good or bad, it straddles both in a political game to keep order. Percy, poor kid, is too naive to see that.
The fact is, he's been used and manipulated by people in power and by the end of book five, when the ministry finally admits that Voldemort is back, well, that changes everything. By Book 6, we get a new minister, but Percy is promoted -- again. The description of Percy in the chapter titled "A Very Frosty Christmas" is one of shame and embarrassment mingled with anger and most notably, disappointment. He can clearly see that the only reason he's made it as far as he has is because the Ministry must still believe he's got an in with the Order, and in with people close to Harry, and they know for sure he has a relationship (however thin) with Potter himself.
The ministry believes it can use Percy in a feeble attempt to control Harry, but it backfires -- twice. I believe the old minister encouraged him to write the horrid letter to Ron in book 5 as a way to control Harry. The language is so over the top, it's hard to imagine even Percy buying it. Then the new minister, knowing Percy is on the outs with his family, concocts some stupid reuse to get him to attend his family's Christmas dinner. At this point, it's clear Percy realizes the only reason they went together was so the minister could get face time with Harry.
In book 6, we see a new Percy, one who's a beaten man, one who realizes he's being used the by ministry. What this means for the next book is hard to say, but a boy growing up in the household he did, knowing that his whole family has been affected by Voldemort (two of Molly's brothers were killed the first time around) well, I don't see him in allegiance with Voldemort ever.
Now, if there is a storyline to be had, it will be between him and the Twins. Those two sides are diametrically opposed: rule follower and rule breakers. This storyline can set up what some see as a redemption for Percy and a way for him to be accepted back into the fold by going against his nature and breaking rules WITH his family in order to defeat either the Ministry's attempts to do something dumb -- again -- or directly aiding in the fight against Voldemort. I'm not sure how this will happen, but that's the storyline I see with Percy, not one that touches the Dark Side any way.
I imagine there will be a very frosty return to the fold. The Twins, Ron and Harry may never fully accept him, but he will return to the family a hero. He deserves that much after how abominably he's been used. Just because Percy's a stick in the mud and a rule follower, doesn't mean he's a bad guy. He really believes what he's doing it the the right thing, but the as the saying goes, the road to hell is paved with good intentions. That will be the lesson Percy teaches us. He will see the error of that judgment and do the right thing.