Have you ever played campaigns where the players were meant to be heroes but were corrupted by outside influences? Evil campaigns where everyone starts from the "dark side" don't count.

IME these sort of games are rare. During my 10 years of gaming I haven't played in any so I decided to have this in my D&D-campaign. The results were a bit mixed.

Two of the PCs were masterless squires. Both were of Neutral Good alignment.
Their Order assigned a knight to test them to see if they were worthy of an accolade. It was very nice that the players were really in to this and the stakes were high: Failing the test would have basically meant that these PCs had nowhere to go within the Order.

After they had successfully passed the test the knight revealed that he had boosted his powers by forming a symbiosis with a parasite called "Gutworm" (Fiend Folio). In his opinion the knights needed to have strong weapons if they wanted to defeat their enemies and the demon parasite inside his body was such weapon. Neglecting his own power might've one day cost him his brothers' lives. Having a Gutworm was a small price for having the power to fight for his comrades and to fulfil his duties.

We had the longest in-game dialogues which were pure gold. In the end the squires turned on each another and the one who had fell to the lure was killed. Fortunately, no off-game drama occurred. What was interesting was that they were still both Neutral Good; The one that fell still believed that he was doing the right thing by allying himself with a demonic parasite since it helped him to become stronger and to defend the weak.

Even though a perfectly good PC died in a PvP fight, we still had one of the brightest moments of dialogue in my gaming history.