I am just nerd enough to get the punchline. I'll explain what I got...

The original joke goes:

Two chemists walk into a bar. The first chemist tells the bartender, "I'll have an H2O." The second chemist says, "I'll have an H2O too." The second chemist dies (because the bartender game him H2O2, which is lethal if enough is ingested)

Now we have hitler chemists walking into a bar. If you know the original joke, then you're waiting for the ending you know. When the second hitler instead says, "I'll ALSO have a WATER" (instead of H2O, too), he doesn't get H2O2 as the first hitler had hoped. So the first hitler chemists failed to assassinate the second hitler chemist.

Did that help or hurt?