Say I wanted to bump off E.T. Would there be any law against it?
If you are a US citizen then yes.
It is against US law to have contact with a being or craft of non-terestrial origin
Title 14, Section 1211 of the Code of Federal Regulations, adopted on July 16, 1969
anyone guilty of such contact automatically becomes a wanted criminal to be jailed for one year and fined $5,000
The NASA administrator is empowered to determine with or without a hearing that a person or object has been "extraterrestrially exposed" and impose an indeterminate quarantine under armed guard, which could not be broken even by court order. There is no limit placed on the number of individuals who could thus be arbitrarily quarantined.
The definition of "extraterrestrial exposure" is left entirely up to NASA administrator, who is thus endowed with total dictatorial power to be exercised at his slightest caprice, which is completely contrary to the Constitution. According to Dr. Clifford, whose commanding officers have been assuring the public for the last 39 years that UFO's are nothing more than hoaxes and delusions to be dismissed with a condescending smile: "This is really no joke, it's a very serious matter."
.Have a nice day