Rodneyse Bichotte Hermelyn, the Majority Whip in the New York State Assembly, tells TMZ that while she’s an ardent animal-rights advocate and sympathizes with Peanut’s owner, Mark Longo ... she believes euthanizing Peanut was the only option, since the N.Y. State Dept. of Environmental Conservation's investigation revealed the squirrel bit someone.

Hermelyn explains to us DEC had no choice but to put down Peanut so they could test the rodent for rabies, which is deadly for 99% of humans who catch it. She also notes it's well-known there are no approved methods to test animals for rabies while they're still alive.

She believes the DEC was just doing its job, noting New York state law requires a license for owning a wild animal ... so personnel were just sticking to the rules and following up on complaints about Peanut not being legally registered.

Hermelyn emphasizes she’s sensitive about the whole situation and hopes Peanut can rest in peace. She even credits Longo for his efforts to legally run an animal refuge, which she describes as "incredibly admirable."

As we reported, authorities euthanized Peanut even though there was evidence he didn’t have rabies -- he had bitten Mark a few times, but he never showed any symptoms.

Peanut had 532,000 followers on Instagram, and Longo says anonymous complaints about the squirrel led state agencies to get a search warrant to rip the animal from his home and kill it. Fred the raccoon was also seized from Longo's animal-sanctuary farm in the same raid and euthanized.

New York Congressman Nick Langworthy tells us he and other members of the state's congressional delegation plan to demand answers from the governor's office and the Dept. of Environmental Conservation.