Kind of a weak case. They had no real evidence, the culprits only got caught because one of them confessed.
I agree, there is a lack of direct evidence. The methods are also a bit iffy.
Had the first victim washed or even wiped his hands before dinner he would have been fine. Also imagine if he had touched his face or anywhere sensitive before the dinner, could have died then and there.
He also must have touched door knobs, car keys, phone, etc., so the poison must have been everywhere too.
Not to mention the perps had to somehow ensure the victim did not notice there is something sticking on his fingers, while there is enough of it to be lethal during the meal…
Similarly, had the second victim fell slightly differently there is a possibility he did not die (though still heavily injured), and he could have testified.