Hello Jack
1. Why would Fred have blown up Jack's restaurant?
Jack's restaurant has been blown up. You can think of no reason for Fred to blow up Jack's restaurant. So you use 'would have', to show that you think that this particular scenario is an imaginary past.
2. Why would Fred blow up your restaurant?
This has two possible contexts.
If the restaurant has been blown up, you can think of no reason for Fred to be the culprit. So you use 'would', to show that you think that the scenario in general is imaginary.
If the restaurant hasn't been blown up, you can think of no reason for Fred to try to blow up the restaurant. Again, you use 'would' to show that the the scenario in general is imaginary.
MrP