Sheriff's Dept -- Justin Bieber Deserves to Be a FELON!

Justin Bieber should be prosecuted for FELONY vandalism for egging his neighbor's house ... this according to the L.A. County Sheriff's honcho who investigated the case.

Lt. Dave Thompson tells TMZ ... it's pretty simple. Justin caused $20,000 in damage, he's caught dead to rights on video, and justice requires a prosecution for felony vandalism.

Thompson -- who executed the search warrant on Bieber's home -- says he'll be one unhappy guy if the D.A. decides to file misdemeanor charges, saying, "I led a team of people into a house where there's armed security. I wouldn't have done that if this was just a misdemeanor case."

And Thompson says, the judge who signed the search warrant agreed it's a felony.

TMZ broke the story ... the deputy D.A. who investigated the case told the victim he believes this is a classic felony case.

We're told the final decision will be made by the top people in the D.A.'s office, including the D.A. herself, Jackie Lacey.