If they wanted to crash a plane with a drone they'd surely make the drone as hard to detect as possible and aim it at something vital like an engine or cockpit window.
I reckon that would be extremely hard to do. I mean, they're reasonably stealthy as it is, but targeting a vulnerable part of a fast-moving aircraft with a slow-moving drone would be difficult to accomplish manually, and if you go to the effort of incorporating a proper targeting system, you might as well throw in an explosive warhead to increase the efficacy. Then there's a strong argument for ditching the buzzy bits in favour of a rocket motor, at which point you've got a surface-to-air missile and if you've got the resources to develop one of *those*, you can probably buy one that works off the shelf from your local shady arms dealer.
At risk of illuminating the purple light at GCHQ, If I wanted to attack a plane with a drone, I'd use it to deliver an incendiary device while it was on the ground. Other suitable targets left as an exercise for the reader.