Drift was a Decepticon during the early stages of the war on Cybortron, because he agreed with some of the political ideas, like all bots should be treated as equals and not given just the job they are designed for (etc. a miner bot can only and forever be a miner).

But he switched side when he saw the corruption of Megatron and his lust for power.

Please don't judge him for his past. Poor bot's been through enough already.