if you listen to the audio, it really sounds like there's another sample triggered for DIE. It's almost as there is a bug in the software that triggers the "DIE!" sample too fast, and this was overlooked by the original japanese devs ..
It was probably planned to say like "Xmen! Welcome to my secret lair" *pause" "DIE!!!" but instead we got "Xmen! Welcome to DIE!!"
Still, you'd think the voice actor would have been like "Hey guys, this doesn't make sense. Are you sure it's right?" He sounds like a native English speaker to me.
I guess it could be another line triggering over it. Hasn't all the audio been dumped via MAME and stuff though? It's certainly a testament to Konami's American branch at the time (or lack thereof).
It's not like it was a low grossing game. They could have revised it.