One of the basic firearm safety principles utilised by...

One of the basic firearm safety principles utilised by professionals is;
1. Know your target and what is behind it.
In other words, are there other persons behind your target.
That principle has to be considered by police for example and can affect a decision whether to fire or not.
The shooter in this incident would have clearly seen a mass of people behind Trump and any rational person would have surmised that any missed shot would very likely have struck innocent people, but he obviously did not care that whether he hit his target or not he was very likely going to strike other people.
Obviously the possible consequences were of no concern to him!