1. Doesn't like to stay in his lane
2. Doesn't like being overtaken so tries to undertake and bully his way in front
3. Cant read signs saying the lane is ending then creates a problem for everyone else on the road
4. Drives a large car to compensate for something
Usually when a roundabout has two lanes on and two lanes off it then it usually means that there is two lanes on the roundabout, not one for you to weave around in.
The next person you cut up might not be so quick with their brakes and then that's a big insurance claim and full liability to you.
Check your mirrors and use your brain!
Ruined a limited-production hot hatch all because they FAILED to use their mirrors and check their blind spot.