I am not opposed to it in most cases for any moral reason, but if we did accidentally contaminate an example of even incredibly simple ET life it would be an enormous scientific loss. What one second example of life could teach us would be massive. Contamination would mean we could never be sure we were studying the authentic thing.
That's kinda impossible though, given the variety of environments we find life in on Earth - there are microbes living miles under the surface, on massively radioactive spent nuclear fuel, inside thermal vents that reach hundreds degrees celcius....in fact I bet there isn't any environment on Earth that doesn't have some form of life in it, other than artificially created by us.
There is an argument that Mars probably already has life on it, as many of the probes we've sent almost certainly brought something with them despite our efforts to sterilize them.
NASA found that some bacteria can metabolize isopropyl alcohol [1]
[1] https://www.theatlantic.com/science/archive/2018/06/bacteria...