I'll write this from the viewpoint of those who want to kill him, as that makes for more straightforware writing.

Here's what we need to do to carry out this killing using the car plan. We need to hack the car to accept some kind of remote control. This will require hacking at least the steering, the engine, and the brakes. We need all three because to ensure killing. If he can control one of them, he has a good chance of making the accident non-fatal.

After we've got the car hacked and can remotely control it, we need to actually cause the accident. There are a couple of approaches here. One is to follow him around in our control vehicle into an opportunity arises. The other is to wait at some place that we know he is likely to go and that is a good place to make a fatal accident happen.

After we've cause an accident, our problem with him is not yet solved. We still have to worry that someone will decide his death was not an accident, examine the car, find that it was tampered with, and maybe tie that to us. Even if it only leads to people we hired and we were competent enough to leave nothing that links them to us, we lose those contractors for future work.

So, what can go wrong with this plan?

First, we've got to do the car hacking. Even if steering, engine, and braking are all "fly by wire" on his car, they are likely to be separate systems. We are going to have to rig all three of them for remote control. We'll probably need to install some hardware for the remote control.

Any suspicious contact with the target before killing him is risky, and right off the bat this plan involves complicated fucking around inside his car.

If we do get his car rigged, then we have to actually take over and cause the accident. The "follow him around hoping for an opportunity" approach is not good. He could go weeks where he only drives in high traffic areas, where we won't get an opportunity to get him up to a speed that ensures a high chance of fatality. The longer we follow him, the more chance we'll be spotted, or that he'll take his car in for service and someone will find our remote control rig.

Our chances are better if we know some place he regularly drives where conditions are better for an accident. Let's assume that is what actually happened--they were waiting for him at the place the "accident" actually happened, and consider how risky that was.

Accelerating up to high speed and nailing a tree seems pretty straightforward. However, it's not as easy is it seems. If you check the photos of the crash scene, on some it looks like the trees are tightly packed, so it would be easy to nail one. However, those photos are looking down the street and mae the trees look more densly packed then they actually are. Find a photo looking from the side, and you can see the trees are much farther apart.

If there is any noticeable lag in the remote control, it will actually be pretty hard to aim at a particular tree. Best you'll be able to do is veer toward the trees, and hope that when the car crosses the curb that doesn't turn it enough to send it between the trees.

Even if you do hit a tree, it will be hard to ensure it is a good solid fatal hit, as opposed to some kind of off center hit that doesn't stop the car but just sends it spinning.

But suppose we get lucky, and actually get a good fatal hit. We've arranged the accident in a place where it will be immediately noticied. We'll have civilians on the scene quickly--before the fire has died down enough to let us get in and make sure any evidence that we want destroyed was actually destroyed. Police and fire and ambulance services will arrive quickly.

This is a Wile E. Coyote plan.

In general, the more steps a plan has that must succeed for the plan to succeed, the lower the reliability of the plan.

Shooting him at an anonymous meeting has two steps that must succeed. (1) Call him from a burner phone with a fake anonymous whistleblower story and arrange a meeting. (2) Shoot him when he shows up for the meeting. This either works, or it fails at step 1 leaving nothing behind to tie it to us and nothing to raise suspicion.