Hello, thanks for your question today! I am Dr. Drew, and I am a licensed veterinarian. I'll be happy to help you in any way I can.
unfortunately, Diazepam, even in very high doses, will not cause the peaceful passing you are hoping for.
Diazepam has a rather high margin of safety, and is unlikely to cause death.
It may, however, cause digestive upset / nausea if large amounts are ingested,
and that could in turn make Bhumipol even more miserable.
I cannot recommend that you attempt an overdose of Diazepam in an effort to euthanize him.