Dave is anything but a well of rational thought, considering his Alex Jones conspiracy bullshit and his constant anger issues. Really, him still being bitter about Metallica isn't all that surprising considering how big an ass he is along with the fact that, despite all it's success, Megadeth is still ultimately a less successful band than Metallica.
And yeah, Dave is certainly a better guitarist than James or Kirk, but Metallica as a whole are still WAY better songwriters than Dave is on average. Besides Rust In Peace (Megadeth's only good album, and one of the greatest metal albums ever, btw), pretty much each one of their albums are more miss than hit, with most of said misses not even reaching the level of 80s Metallica's worst. Like, you could cull all the good tracks from Killing is My Business, Peace Sells and So Far So Good So What and maybe get a solid 30 minute album that's about as good as Ride the Lightning, but that's about it...
(And yeah, I know that Dave contributed to the writing of Kill 'Em All and Ride the Lightning, but considering the gulf in quality between Megadeth and post-Mustaine Metallica, I have to question how much he actually contributed...)