In the meantime one of the guards sprints to attach him.
The player was not satisfied of this, because he got someone who tried to killed him like if he rolled 6-.
My guess is that it doesn't matter, the success did not give him a sort of immunity.If someone want to kill him because of the fiction he will be attacked even with a success.
Am I right or I've been a bit too harsh?
To give a dissenting view, I'd say that except in some very particular circumstances I wouldn't have done that. If, say, it had been previously established that the guards were going to attack anyone who gets in their way, or if they're under orders to kill the Outlaw Heir on sight, or if the Heir had struck down one of the guards as part of their flashy entrance, then sure I'd say that "one of the guards steps forward, sword raised" is a valid response, but if all that happened thus far is "As your ruler by birthright, Ossumman Stone III, I command you to give these people the taxes you've taken," and the move wasn't a 6-, the most I'd do in that direction is have a guard come up, sneering, hand gripping his sword that's still sheathed, being all, "Uh, firstly, no, and if you say anything more, we'll be getting all excessive force up in this bitch." That's when the Heir is given an opportunity on a goddamn platter to Claim Their Rights ;)
As far as I'm concerned, you were too harsh (unless previously established that the Heir was to get the shiv). That said, it's your and your friends' game, and the only people who can really judge are you all. Talk to your fellow players. Say your piece. Be open to being wrong, but stand by your decision unless discussion convinces you otherwise.