Be my guest is an everyday English expression which does not literally mean that you are inviting somebody to be your guest, but are sarcastically implying that the person can go ahead with whatever he / she intends to do.
The everyday English expression Be my Guest is conveyed in a brusque manner which conveys that you are annoyed. It could be said either in a mild manner or in a loud angry tone.
1. Kiran's lawyer told her that the police had a valid search warrant and she could do nothing to stop them entering her home. So she stepped away from the door and allowed them in saying "Be my guest. Just don't break anything!"
2. After getting repeated calls from his fiancée, Shyam exasperatedly said "Ok, fine, since you are so determined to have your way, be my guest and go ahead with your plans. Anyway you are not going to listen to me!"
3. The neighbours could hear the Mishra brothers quarelling, the older brother loudly yelling "Be my guest and do what the hell you want! I will not give in to your threats"