This whole page makes for fascinating reading:

http://www.projectrho.com/rocket/rocket3aa.html#killingstar

Keep in mind that they guy who made this site also designs RPGs for
space battles, but his comments still stand, assuming ETI that are not
angelic beings living in another dimension are either similar to us or at
least evolved along similar patterns to life on Earth.

If ETI wanted to get rid of us, they could either slam a starship into
Earth at relativistic speeds as listed above, or drop a bunch of space
rocks on us and wait for the dust to settle. There won't be fleets of
ships with alien troops aboard, because they won't be needed to get
rid of humanity at this level of our development.

As for what would insult an alien, check out Carl Sagan's book Cosmic
Connections from 1973. Discussing the Pioneer Plaque, he quotes an
article from the old humor magazine punch about how different species
who might encounter the Pioneer probe and its golden calling card may
react to it.

The one which I recall had creatures that resembled the pulsar map on
the plaque and thought its positioning was in the rudest gesture possible
for their culture.

Omni magazine did something similar in a story from 1978 regarding the
Voyager Records. The punch line hinged on one of the UN greetings
asking any aliens if they have eaten yet.