None of this is an attenpt to justify his terrible actions, but simply a plea for an explanation:
Although some form of mental disorder is very clearly present here, I would venture that it rarely occurs in a vacuum. Social and environmental factors play a role in conjunction with genetic predisposition.
It is truly striking that this boy went from being a promising talent at 11 to what he ultimately became. Perhaps the 'promise' was not fulfilled as he may have hoped, leading to frustration, which in turn affected his mental state. Adolescence is a complex time where expectations do not necessarily materialise and childhood dreams can be dashed.
Inclusive advertising is prevalent nowadays, but it is unclear whether individuals featured in this way are meaningfully able to access further opportunities as a result or whether these are largely one-off stints.
I have observed a couple of cases of how children of domestic servants of wealthier households in a developing country can be particularly vulnerable to mental illness as they are exposed to 'the life they might lead in an alternative reality' while nominally being treated as equals or receiving charity from their parent's employer (paying for food, school fees, etc.), but which in practice creates an even wider gulf vis-à-vis their actual living conditions (in shanties or rural poverty) and realistic life prospects, in the absence of the necessary social capital or all-round support required for genuine success along the 'promised' lines. The challenge can be too daunting for some and they often break, succumbing to psychosis, etc.
Against this family's painful background of fleeing genocide in Rwanda, they may well have been pinning their hopes on young Axel. But the pressure may have been too great to bear, taking into account his perceived life prospects as he entered adulthood. Appearing in the BBC advert may well have been his 'high point' with things gradually going downhill from there (it happened to Macaulay Culkin after Home Alone, not to mention many other child actors), with the 'promise' ebbing away and resentment brewing precisely against what he once held dear – to the point of seeking to annihilate it.
In anthropological terms, the broken promise of equality is a historical driver of violence as David Graeber notes in his book on 'Debt'.