Andmurdereris a personwhointentionallykillsanotherperson.
SowhenMortysays, yousold a gunto a murderersoyoucouldplayvideogames, heisexpressingdisbeliefanddisapprovalthatRickisputtinghisownfunabovetheharmthatmightcomefromsellingthegunto a murderer.
Soaltogether, thephrasehindsightis 20-20 is a commonexpressioninEnglish, whichmeansthatit's easytoseewhatweshouldhavedoneaftertheeventshavealreadyhappened.