even himmler didnt had the stomach for it to see the jews being shot and insisted another way of dealing with them like gassing.
hmm, thats not 100% accurate, he apperantly witnessed one of the many mass shootings at the beginning of the ethinic cleanising in Latvia Riga, Himmler was a person that could faint or feel very unwell when seeing blood, so he was pretty shaken.
he didn't demand a different way of killing because of that, but because his Einsatz kommanders adviced him that it was a) Very labor intensive b) They simply wasn't enough space to do all the killings c) not enough time and last but not least it had a very very lasting effect on the men that did the shooting, or at least on many.
So to clarify the reason for changing to Gas was not driven because Himmler coudln't stand the sight of blood but because of simple economical reasons as so many other things in the third Reich it wasn't practical.
Take the Gipsys, they were plans at the beginning of the third reich to make them "sesshaft" but the whole project became to complicated, so they simply decided to kill them all instead - again the same reason behind mass slaughter,.