Is Magic Lantern harmful for the dslr?

It is for Videos (in low light especially) [ I don't like the camcorder video quality ]
Will a canon 600d with magic lantern work properly? The ML website says that it can destroy the camera. Is it really harmful?
Else I have to go with the nikon d5200 because it performs better.

You can potentially damage the camera if the firmware installation is interrupted (power outage, battery drain). The ML firmware may also have fewer safeguards than the Canon firmware, or unchecked glitches that may prevent the camera's firmware from being repaired or replaced.

You are much better off learning how to correctly light a scene for video, and possibly buying a studio lighting kit. "Dark" scenes in movies may have brighter lighting than you expect, since the camera performs better at low sensitivity. Actually shooting in low light is for amateurs who have neither the resources to alter the scene, nor the patience to do proper videography.

With proper lighting, even the camcorders might take agreeable video.

"I don't like the camcorder video quality" - have you compared apples to apples, as in 1080, 30p in both a DSLR and a camcorder?

"Will a canon 600d with magic lantern work properly?" - It can.

"Is it really harmful?" - It can be.

"Else I have to go with the nikon d5200 because it performs better." - While I admire Nikon, in general, I don't see any advantage of the D5200 over the 600D. So, you've made the claim, prove it.

I would not trust it. Especially when the Magic Lantern website itself discloses it can bust your camera. And I'm not too sure it's going to fix whatever image-quality issues you perceive as those limits are more hardware than firmware.

I agree. Regardless of what you've heard the ML site carries a warning that your camera may not work at all with ML installed. I would take that warning seriously. If your firmware is damaged then it's back to Canon for a re-install and that will be expensive.