There are valid arguments against the fire, but a reliable witness saw fire in the sky so something has to explain that, at the very least an engine fire before something else happened.
I haven't read about this. Was this witnessed around the area where the plane turned around?
Equally I've wondered why a man who has killed his co-pilot, cabin crew and passengers flies on for 6 or 7 hours for a belated suicide. Usually people just get on with it.
It's not really possible to kill everyone that way, at least not quickly. The oxygen masks will drop, so everyone will be fine until the oxygen supply runs out. Then, the cabin crew have additional oxygen cylinders they can breathe from, I think those last for hours.
And the locked out co-pilot will try everything during those hours to try and break into the cockpit...
It's an exceedingly long anticipation of his own death, all while dealing with the enormity of what he has done, unless of course he committed suicide early and the plane flew on until it ran out of fuel, stalled and nosedived into the ocean.
If it runs out of fuel the plane won't nosedive, it will glide, gradually losing altitude. If one wing runs out of fuel before the other, then it will enter a spiral, but still not a steep one.
But set against that there are some claiming it had a piloted landing in the ocean.
I don't think it's possible to do a piloted landing in the middle of the night in an unknown place. Even if he used a personal GPS device, he wouldn't know the local QNH so the plane's altitude information would be inaccurate meaning, he won't know how far the ground (water) is, and wouldn't know when to flare.
The gist of the argument for murder-suicide is this: several calls and messages to the cockpit went unanswered. But the plane continued to make turns. So, most probably, someone in the cockpit was making those turns. While theoretically it's possible that the autopilot has been reprogrammed, if you're incapacitated, then won't be able to do that. And if the pilot reprogrammed it before becoming incapacitated, why?
An engine fire is not a big deal. Just turn off the engine, put out the fire (built-in fire supressant system). And then declare an emergency and request to land either in Vietnam or Malaysia.
A lithium battery fire in the cargo is much worse, but again if that was the case how did the plane continue flying for 7 hours?
As for aliens.. did they break the plane into pieces first? Some luggage washed ashore so the plane must have broken up.
Here's another opinion, I think it's well researched: https://www.quora.com/What-might-have-happened-to-the-people-traveling-in-MH370/answer/Sy-Gunson