Fritz Haber is typically cast a tragic figure. No one is sainting him, but typically his name is associated with the Haber process for making ammonia and only occasionally with chemical warfare.
Haber was Jewish and born in Prussia. To fit in and be accepted by his peers, he converted to Catholicism. During WWI all of the greatest German scientists signed 'The Manifesto of the 93,' which was a piece of propaganda justifying Germany's position in the war. The social pressure on Haber to support the war must have been enormous, and he probably saw it as a good chance to prove how much of German patriot he really was. Hence he came up with the idea to use gas chlorine gas in warfare, and it was used successfully in the Battle of Ypres in Belgium.
On the allies side was Victor Grignard. During WWI Grignard created phosgene, which is much more deadly than chlorine gas. The better comparison then is not to Oppenheimer's Manhattan project two decades later, but to Haber's contemporary chemical warfare opponent Victor Grignard. Grignard is much more rarely associated with chemical warfare in spite of phosgene being responsible for many more deaths than Haber's chlorine gas.
One explanation is that Grignard was on the side that won and Haber was on the side that lost. More fairly, Grignard's phosgene was a response to Haber's chlorine gas. Thus, Haber accurately gets more credit because his action could reasonably be viewed as the birth of modern chemical warfare.
With respect to the Manhattan project, it was a very different weapon . The Manhattan project pooled together the greatest minds in the world to create a weapon capable of leveling entire cities. In WWII all of the death and destruction caused by the atomic bomb ended the war. The end of the war was a good thing. The death of hundreds of thousands of people was a very bad thing. As such it is usually discussed with much moral ambiguity.
In the instance of WWI, chemical weapons were just other methods for mutilating the enemy in the normal back and forth of trench warfare. They were not a determining factor in the war, and when they weren't lethal they left soldiers maimed, sick and incapable of returning to their lives in any normal capacity. There's not much moral ambiguity here; chemicals were just another disgusting weapon that if anything prolonged the war and led to worse injuries than had they not been invented in the first place. Thus, there is little moral ambiguity, which is why the Geneva Conventions outlawed the use of chemical warfare after WWI.
If anyone demonizes Haber more than Oppenheimer and the other scientists at the Manhattan project, it is probably because of the lack of moral ambiguity surrounding the use of chemical warfare.