IF he is guilty (and he is currently innocent as he hasn't been proven guilty at the time of writing), then the first thing I imagine is the following. I could be totally wrong, it is very disturbing and I am sorry to share this possible scenario with you:
Possible scenario: The very young 6 year old autistic child is screaming, shouting and doing many things autistic children often do when they're upset or perhaps even when just playing, the 19 year old Uncle being sensitive to certain noises gets more and more worked up wanting quiet and the noises were particularly sensitive to him. The uncle could have had a sensory overload and shouted back at the child which would have only made the child worse and this would have in turn escalated in a vicious circle making the Uncle's sensory overload worse to the extent that eventually he just couldn't handle it any more. The Uncle's sensory overload could have suddenly turned into complete meltdown and then in an altered mental state he could have done what he thought he had to in order to get the 6 year old to finally "shut up", E.g. hold the child underwater (he may not have even thought this would kill him, but held him under too long before he came to his senses). Afterwards he would have realised what he'd done and would have severely regretted it, he would have almost certainly panicked and then tried to cover up the crime fearing what would happen to him if he was convicted of murder (he would have done this very poorly, not thinking straight in a terrible panic, I can only imagine the terror he would have felt in this dire situation).
I have sat in my flat listening to my neighbour banging about upstairs, it can be very difficult to handle sometimes and I can feel anger towards him even though I have to keep telling myself that he's not really doing anything wrong and it's me that is over sensitive to certain noises. I have had meltdowns where I've felt like going outside, smashing down his door and attacking him, I wouldn't do that and would instead walk away somewhere to calm down (I've never been violent in this situation), but the thought has certainly been there, also perhaps the Uncle thought about walking away and was torn between his responsibility not to leave the young child alone. I'm sorry if this isn't what people on this forum probably want to hear and it's not what I want to conclude either, but IF he did commit the crime, then it could have been at least partially caused by him being on the autistic spectrum.
Update:
I have a further thought. IF my scenario did happen to be close to the truth it would be very difficult for the law to deal with the crime. In one hand the uncle wouldn't truly be an evil person, he would be extremely remorseful and would have mitigating circumstances for committing the crime. On the other hand however the law can't be seen not to apply a severe punishment for killing, especially a child and if the uncle received little punishment then it could be perceived that autistic people aren't always responsible for their actions and could even kill during a meltdown and get away with it in future. In other words the barrier to stop people killing must stay firmly in place and autistic people must realise that even during a severe meltdown they must avoid violence and especially killing at all costs.
If he was unable to handle a young autistic child, then it should also be questioned why he was put in a situation where he was babysitting one. The problem is this case could well make people generally untrustworthy towards autistic people being around children even when the vast majority would be perfectly fine and it's unfair to be prejudice towards people based on what others have done (we are all individuals). The whole affair is a horrible can of worms.