Last month former Iron Maiden vocalist Paul Di'anno was in Argentina doing a press conference.  At some point the interviewer asks Di'anno about the rumors of drug use being the reason he was kicked out of the band.  Paul vehemently disagrees and even compares Steve Harris to Hitler at some point.  The video below is of the press conference and at around 6:25 into it, is where Di'anno answers that question. 
Here's an excerpt:
"Where the fuck do you people get this from? I left Iron Maiden because they were going too heavy metal, and Iron Maiden is a money-making machine, and I don't give a fuck about it. It was not about drugs; it was nothing like that. Me and Steve... I wrote the song 'Killers', Steve thought he had better songs. I thought his songs were shit. "