I hate Trump and everything he stands for, but America has been like this for a long time. Years ago, before Trump ever thought of running for president, a British tourist who got lost in Florida was shot dead when he approached a house to ask for directions. The home owner was not convicted of any offences. This is not a solitary incident.
The argument about Trump and the shooting appear to be separate events. The father says the shooting was an accident. The grand jury, in a state which has some of the most permissive gun laws in the US and where gun ownership and support for gun ownership is high, decided there was insufficient evidence to prove it wasn't an accident.
If this had happened in the UK, I think the father's own description of events is sufficient to support a manslaughter charge. In the US, however, supporters of gun ownership are more likely to regard this as an unfortunate accident.