A thought experiment: is evil research justified?

let's say that we have two two chefs. 

one chef is a convicted felon and a murderer while the other is just a normal public citizen. 

both chefs sell sandwiches that they make; the convicted murderer sells the best sandwich that you could ever eat and it's rated as the best sandwich on the planet. while the normal citizen chef sells a sandwich that tastes so bad that it makes people sick. 

which chef would you consider it being okay to order a sandwich from? would it be wrong to order from the convicted murderer because they're a murderer? even though they sell a genuinely good product that is the best of its kind? or would it be wrong to buy from the chef that genuinely makes people sick when they eat from them? and let's say that you have to choose one of them to order a sandwich from. 

using the previous established lore, let's say that each chef has their sandwiches swapped. let's say that the convicted murderer is selling the sandwich that is making people sick and that the normal citizen chef is selling the sandwich that is the best on the planet. who would you order from then? and now let's add in a surprise Factor, the two chefs have actually stolen the recipes from each other. 

considering the answer that you just gave, would you continue to stand by that answer and not falter? would you still order the delicious sandwich from the normal citizen chef, even though that chef stole the recipe? and even though the recipe was stolen from a convicted murderer? this is important because now both chefs are known felons and neither one of them is an innocent party; both have stolen and committed a crime but one has committed a greater crime. 

the reason for this thought experiment? is it worthwhile to buy a product from someone when they are a genuine horrible person or they've done terrible things, if that person has a genuinely good product or has done genuinely incredible and valuable research or gathered valuable information that the world could use? 

now this might upset a few people, in fact it's going to upset a lot of people, and this is not my intention at all. I only mean this to give people a simple discussion. but, in world war II the Nazis did many MANY MAAAANNNY horrible and terrible things to the people that they kidnapped and tortured and eventually murdered. I in no way condone what they did, because what they did was horrifying and terrifying. however we would not be where we are as a society or as a planet without much of the horrible and taboo experiments that were committed during world war II.

 regrettably, the world decided to use the research, in spite of its ethically questionable means and motives. and in spite of the very many people who were killed. and now we know a lot of information we would have never known, like how twins interact with each other. and how an infant child will die if left unattended or shown any love. 

the thought experiment I'm posing to the world today, is whether you think, and this is to YOU personally reading this, it is okay to use information or products obtained and created, even if those products are created by terrible people, if those products and that research could genuinely benefit the entirety of humanity or are a genuinely good product? 

i.e. if a murderer literally did make the best sandwich on the planet, would it be worth buying that sandwich? what are your thoughts?