Jesus said disaster happen to christians too in scripture when the building fell on believers.How do I trust God if he let bad things happens to believers?
You've asked a deep question that has derailed many.
For what it's worth, here's how I look at it.
God gave mankind freedom to make choices. Unlike animal instincts, mankind seems to have been given a higher kind of thinking.
So someone build a tower. Somewhere they made an error, undercompensated, or maybe even cheated on the engineer's design and used cheap screws instead of lag bolts or the equivalent. People near the tower were crushed when it collapsed, simply because that's where they were. It probably wasn't their fault at all.
Or someone with an uncontrolled temper or lust abused a child. It wasn't the child's fault at all. Or those Galileans whose blood Pilate mingled with their sacrifices? It probably wasn't their fault either.
Where is God in all this? You've asked a good question.
If God legislated our behavior, like whether we got up at 5:30 AM or at 7 AM, whether we were leisurely on our way to work or in a hurry and apt to make an error in judgment while driving and cause an accident - well, you've probably noticed that it doesn't appear that He does much of that like of legislating. It looks to me like a lot of stuff just happens as a pile up of human choices. God already told us what we need for life's choices to minimize the pile up of mistakes. I can imagine you know as many people as I know (including the person looking back at me in the mirror) who far too often live haphazardly and "hopes" for less than I/we deserve. If we could only grasp the depth of His love and the love He wants us to have for one another, we might just be more conscientious and actually obedient to what He says works in life. Besides that, some people seem to delight in cruel and evil behaviors. At this point, God doesn't generally stop them. He allows them choice to show what kind of people they are, just like He allows us.
Were those people in the tower worse sinners than we are? No, but unless we repent, we will all likewise perish. And in the process we'll leave a bigger pile of errors and junk to trip up even more people.
Where is God in all this?
He hasn't left those who put their trust in Him. Though they were killed by sword, sawn in two, killed by "accident," or died by natural causes, He is there. God has a bigger plan in mind than just here and now, to the point that it can be like this for now. If there is some kind of expectation that God won't let your foot dash against a stone, it might be good to look at Jesus, who for the joy set before Him, endured the awful death by crucifixion, enduring the shameful treatment, and is now at God's right hand. Most of the disciples suffered death at the hands of murderers. We in America for the most part have never even imagined this kind of persecution, yet they maintained faith and I hope we can too.
These are just my thoughts and I hope they might be of some help to you, Friend.