He was obviously serious because an army cannot fight without a commander. 2 Kings 10:3 is a Hebrewism for an invitation to war.
2 Kings 10:3. Fight ye for your master’s house — This he said,
because, perhaps, he had received notice of their intentions to do so,
and that they were consulting about a successor to Joram: or to try
them, whether they would do so, or would be true to him and his
designs: or, which is still more likely, he insults over them, and
denounces war against them with scorn and derision, being secure of
the event, and knowing that they were not able to oppose him.
The pulpit commentary observes:
Verse 3. - Look even out the belt and meetest of your master's sons,
and set him on his father's throne. "Choose," i.e., "among the sons of
Joram the strongest, the boldest, and the ablest, and make him king in
his father's zoom; take him for your leader against me; do not
hesitate and beat about the bush; but at once make up your minds, and
let me know what I have to expect." And fight for your master's house.
There had been a civil war before the dynasty of Omri succeeded in
settling itself on the throne (1 Kings 16:21, 22). Jehu believes, or
affects to believe, that there will now be another. He does not
deprecate it, but invites it. Probably he felt tolerably confident
that the garrison of Samaria, even if called upon by the municipality,
would not venture to take up arms against the army of Ramoth-Gilead,
which had declared itself in his favor. Still, supposing that it did,
he was not fearful of the result.