In the last scene of this gif Ned throws a body out. The end of it is seen here:
What's going on? The rest of the gif shows that he's doing regular stuff, as seen in the full video:
The gif you refer to is from Treehouse of Horror XXII, and in particular the section Dial D for Diddily. To quote from the wiki:
In a parody of Dexter, after hearing who he thinks is God tell him to murder people, Ned Flanders becomes a serial killing vigilante, targeting characters who are Homer's enemies.
Specifically, the gif is a parody of Dexter's opening sequence, which shows Dexter's morning routine in grotesque close-up that alludes to the show's more gory aspects. Dexter cuts himself shaving, we see the blood fill the screen. Dexter cuts ham, we think of his plastic-wrapped victims. Dexter squeezes a blood orange, etc. The Flanders gif flips it. We see a close-up that appears really sinister, then turns out to be totally harmless. That's humorously subverted again in the final clip, which is just gory through and through.
To quote from the wiki on The Simpsons Treehouse Horror:
These segments usually involve the family in some horror, science fiction, or supernatural setting and always take place outside the normal continuity of the show and are therefore considered to be non-canon.
This is quite obvious given how many non-canon things happen in each of the various Treehouse Horror episodes.
So Flanders murdered someone in a non-canon, supernatural episode, not in the "normal" continuity of the series.