Originally Posted by
eschatological
I had to re-watch these scenes to see if what you said made any sense. It didn't.
Jaqen is clearly a guard, and Arya asks him to kill people he's on cordial terms with. She asks him first to kill the Tickler, and the scene cuts away......some time has passed, the Tickler shows up dead, his head twisted backwards on his body, below a window which Jaqen is sitting in. The implication here is not that Jaqen stealthed up on him in the courtyard, then magically teleported up; the courtyard is out in the open, in the middle of the keep. Jaqen worked with the Tickler. He probably observed him (like the FM do), found him in a dark corridor with no witnesses, broke his neck, and threw him out a window. That seems to be the reasonable interpretation of what happened off-screen, not your weird "he's a sneaky assassin" extrapolation.
Then Arya names a second name, and she wants it done now (which should be reasonable for any sneaky assassin), and Jaqen responds, "A girl cannot tell a man when exactly he must do a thing. A man cannot make a thing happen before its time." Arya insists on now. The scene cuts away, and cuts to Lorch falling through a door, dead, a weapon with Lannister red sticking out of neck - the man is also a fellow guard, and Jaqen need not sneak up on him at all, just get close enough to stick him with the pointy end.
Then Arya wants to know how long it'll take for a third name. "A minute, an hour, a month," says Jaqen. "Death is certain...the time is not." She demands he kill Tywin Lannister, and Jaqen says he cannot do that. Then she asks for the guards to be killed, something he achieves offscreen, and again, the reasonable explanation is that HE, as a guard himself, just walks up to them and kills them. We never see it.
I feel like Jaqen's insistence that names can take a month (presumably of observation, of being "seen but not seen" and then striking quickly) or more, and the religiousness of his "the time is not come" stance indicates to me that he's not a sneaky stealth assassin, but a magic-using disguised assassin who ingratiates himself into the lives of his victims to make them let their guard down. A thing he could achieve in HArrenhal easily because he was conscripted into the guards already.
And again, I feel like my representation (that he used his status as guard) is a much more reasonable representation than yours, that Jaqen needs to sneak around to kill people. Even if this is an edge case (I don't think it is), it can be interpreted multiple ways, and in hindsight, a reasonable observer who now knows the tactics of the FM from Arya's training, must conclude that's how he did it.