How does a young person (any terrorist, not just this one) become so disillusioned by life and so filled with hate, that killing many others in your suicide is an option.
Most of us here can state......My child would never do this.
How do we know?
Those who are of the view that Muslims should be denied entry to the US. I ask with all due respect. Will you stop them visiting as well?
Guns are easily accessible in the US for those with a little cash to splash.
A terrorist could slip in amongst the millions of tourists each year and still kill many people.
Or worse, you will lose the tourist dollars, student dollars, and healthcare dollars of legitimate Muslims who visit.
How will you know who the Muslims are?
When family and friends are made to feel unwelcome in your country, US born Muslims may leave, taking their skills with them.
Do not expect to be welcomed in any Muslim country in country tit for tat.
When you have rid yourself of all the Islamic terrorists (and Muslims) take a look at your annual mass shooting numbers, they may fall to pre 2001 levels. You will now be free to work on homegrown mass shooters.
The only way to stop the attacks is to stop young people becoming radicalized, to do that you will need to understand how and why it happens. This involves a dialogue, this will be difficult if there is no one left to talk to.
Government has to play their part too. There are as many answers as there are questions, but your country isn't able to lock the windows, switch on the alarm system and keep the world out.
BTW. If every country that had US immigrants in their penal systems decided to deport them in reciprocation, you will need a few more jails.