The joke is, that the owner had no clue about it's dual-use. But he already suspected that something is wrong with his gardening bot (maybe because of some murders). And now it comes for him.
If a tweet (or similar) makes a statement of fact, either mark it as "satire" in the body or provide the original source link backing up the statement (see rule 6)
Is the joke that he tried to kill someone with the bot?
The joke is, that the owner had no clue about it's dual-use. But he already suspected that something is wrong with his gardening bot (maybe because of some murders). And now it comes for him.