'hitsu' means 'never fail', 'satsu' means 'kil.l', thus 'hissatsu' literally means 'kil.l without fail'
but if someone say 'hissatsu...' it does not necessarily mean 'im gonna kil.l you' unless the person is superhero and you are the villain XD
in most cases that means 'the greatest technique(move, performance)'
also 'hissatsu' is the name of tv drama series (one of the longest-running series, since 1972!), so many people recall 'hissatsu series' when they hear the word