Lockdown began as an Empty
, a Cybertronian who could not transform. He was one of many in a more or less peaceful region of Cybertron, one relatively undisturbed (sort of like the Amish, when you think about it) At sone point, there was an uprising on Cybertron; revolutionaries (not the Decepticons, they hadn't been founded yet) were trying to persuade the Empties over to their side. But being Empties, they were hesitant to have anything to do with 'Metamorphs'. Lockdown's view of the revolutionaries was sketchy at best; on the one hand, they were Metamorphs and clearly had made dangerous enemies; but on the other hand, they also promised a haven for the Empties once the revolution was over.
One day, a young Megatron (then 'Megatronus' and still a good guy) stumbled upon the Empty lands with a bad case of amnesia, he then met Lockdown and they became friends of sorts, him forgetting that he could transform. Locks consulted Megs on the situation with the revs, and although Megs was memory shot, he didn't trust them and advised Locks not to as well. Later one of the revolutionaries recognized Megs and exposed his metamorphic abilities, also proclaimed that he was one of the revs' greatest enemies.
The Empties attacked Megs, forcing him to fight back. When he blasted one through the chest, Lockdown -having known the Empty Megs had just killed, intervened to try and quell the fighting. But instead Megs wound up blasting Locks instead. Megs got away and his memories back back; but Locks was left by himself, never wanting to have anything to do with Megs or his allies, but now also shunned by his own people for trying to help a metamorph. The shot had severely injured him, giving him scars that never would heal. Embittered, he left his homeland and went to the revolutionaries, offering his services on behalf of his people, since they'd severed any ties with any metamorphs -including the revs.
Instead, he given to their head scientist -Scorpanoc, while they battled the legions of Sentinel Prime in the Empty lands. Scorpanoc experimented on Lockdown; rewriting his physiology and cloning him with his experimental protoforms -Scorp had a thing for drones, so hence that Scorpion drone in the movies. Eventually Locks broke free of his restraints and turned on his captor and torturer; after a grueling struggle, knowing his people had been slaughtered in the crossfire, Locks had one of Scorps' own scalpels ready to be plunged into his head.
But before the killing blow could be made, Locks was interrupted by the appearance of a bounty-hunter named Devcon
[no relation to this Devcon
] Scorps had made quite a few enemies, and at least three bots had hired Dev to bring him in. Alive. Not wanting to lose his bounty, Dev offered to help Locks adapt to his now meatmorphic body and make use of his clones; reluctantly, Locks agreed.
But though reluctant, he made good use of his training; once adapted to his new form and minions, Lockdown became a bounty-hunter himself and has been catching the wanted ever since. One of his more recent catches happens to have been one of the late Devcon's informants; Slizardo
(this same slimy character is the one we saw 'affectionately' tasting Tessa's leg in the film)
You can see him in this video
at around 5:38