The principal reason that Hitler did not use chemical weapons on any large scale on the battlefield was deterrence. By the time he accepted that Germany was losing the war, the Western Allies had air dominance over Germany, and could have attacked German cities with gas. They had the weapons available, were prepared to use them in response to German chemical attacks, and this was known to Hitler and the OKW.
By this stage, in late 1944, the Luftwaffe could not effectively resist the bomber fleets, and had no ability to attack Great Britain with gas in response. The V-2 wasn't suitable for gas warheads, simply because it lacked a proximity fuse.
Further, the Germans assumed that the Western Allies had discovered the family of organophosphorus nerve agents that includes sarin, tabun, etc. This was actually incorrect, but was perfectly plausible: they'd been discovered by the German chemical industry during insecticide research, and they knew the USA and UK had worked on that.
Source: Germany and the Second World War, Volume V/IIB, pp760-772.
There had been German military casualties from German use of gas during WWI -- with the limited protective equipment of the time, a few were inevitable.