technically HE didn't steal it. he was the original writer of the song and was originally in metallica, but was kicked out due to drug abuse, and the guys pretty much stole all ideas for his songs, and so mustaine just used his old song despite metallica already coining that riff. but the guys didn't even sue him by the time mustaine made mechanix. they pretty much said "well...it technically IS still his..." so, technically hetfield stole it.