if you are versed in star wars lore. IG units have been a thing for a long time, as have assassin droids. In-universe they are prohibitively expensive to make and hugely regulated. I would bet that in universe it would have been cheaper to hire all the cannon fodder that the IG unit gunned down than it would be to create a new droid. Besides had that IG unit been there alone he would have lost.

Side note, As the mandalorian will continue to explore there is a real distaste and distrust of droids following the clone wars.