Really? we carved up Africa, using arbitrary boundaries
I assume the 'we' you are referring to is the Berlin conference where the Germans, Italians, French, Spanish, Belgians, Dutch, Austrians and yes the Brits agreed on the various sphere of influence.
we allowed these dictators free rein because they carried on supporting the UK via the commonwealth.
Not really, Bokassa was dictator of the central African Republic, a former French colony, and Mobutu was president of the DRC, a former Belgian colony, neither were in the commonwelth. Most former British colonies willingly signed up to the commonwealth, an organisation that did not offer much in any real sense to either Britain or the former colonies.
We stood by and allowed them to screw over their peoples
The strong men dictators in africa tilted to either Russia or the USA in the cold War power plays with the benefit from either camps being access to arms and finance. Anyway, having just been 'kicked out' how would you propose that a rapidly fading former colonial power that had been under huge global postwar pressure to give up its colonial possessions intervene to remove belligerent dictators without the predictable accusations of meddling or neo colonialism or putting boots on the ground.
the UK and other colonial powers are equally responsible for the mess we left.
Which is a bollocks statement really. In Africa British colonies by and large transitioned to independence via the ballot box and formal timetabled handover as a result of both internal and external pressure by various independence movements. French handovers were a mixed bag with peaceful independence gained in the likes of Senegal where as Algeria was a result of violent rebellion. Chad and Car were pretty much abandoned and bottom of the pile were the DRCs and Guinea Bissaus of this world which were stripped bare of any manufacturing base when the Portuguese and Belgians left leaving them starting from scratch.