In Beast Machines, Silverbolt's Spark was placed inside a Vehicon drone. It was harshly corrupted. The Vehicons programing overwhelmed Silverbolt's heroic nature and fully corrupted him. Eventually he lost all of his memories.
It wasn't until Blackarachnia's intervention that he was able to remember who he was. After Blackarachnia successfully freed his Spark and gave him a Beast Mode, Silverbolt was free of the Vehicon's corruption and rejoined the Maximals.
Though no longer evil, Silverbolt struggled with what he had done.