It actually is enough to qualify as being a villain. Stealing, and doing and selling drugs are villainous acts and thus make the accuser a villain.

He's a protagonist villain. He's done more than just steal and do heroin, he shoots dogs for laughs just to see them attacking their owners, he led his friend Tommy onto heroin addiction and HIV infection for money something that led to his death (Which to be fair he did feel bad for but that doesn't excuse it). Lastly he robbed his "friends" of £12,000 regardless of how morally inept Sick Boy and Begbie are that is MASSIVE amount of theft and betrayal, that alone is enough for him to qualify.