Just because others have seen the warning about Loki does not mean that everyone has as you just pointed out that you yourself missed it. Add in there are people here currently who are from a time when there are not these threats around, it is not hard to believe that mistakes are made. They are from a different time when caution is not as necessary and that is not their fault.
Also, you seem to have missed the fact about Loki being one of the good guys until recently and if that warning message was missed, why would anyone think twice of talking to him? Especially with people being switched back and forth at random and no way to keep track of it.
Mistakes were made. And if someone is out to hurt people, it does not take much 'egging on' to make them act. We do not know his motivations. All we know is that he has hurt three people now at the very least and will continue to do so. Perhaps now people will realize not to talk to him as he has outed himself in regards to the explosion, but that will not stop him. People like that do not stop.