In that episode, Picard clearly acknowledges that what Worf did was legal under Klingon law.
PICARD: Mister Worf, your service aboard the Enterprise has been exemplary. Until now.
WORF: Sir, I have acted within the boundaries of Klingon law and tradition.
PICARD: The High Council would seem to agree. They consider the matter closed. I don't. Mister Worf, the Enterprise crew currently includes representatives from thirteen planets. They each have their individual beliefs and values and I respect them all. But they have all chosen to serve Starfleet. If anyone cannot perform his or her duty because of the demands of their society, they should resign. Do you wish to resign?
WORF: No, sir.
PICARD: I had hoped you would not throw away a promising career. I understand your loss, We all admired K'Ehleyr. A reprimand will appear on your record. Dismissed....
from Episode 81 ("Reunion") transcript at chakoteya.net.
It's explicitly not murder as Klingon law provides provisions for when revenge killing is lawful. As @Valorum states, there is probably some level of mutual respect on the part of the Federation toward Klingon culture that tempers the reaction significantly. In this case, Worf is not a fleeing international criminal or a fugitive from Klingon justice, just someone with a little need for further cultural assimilation.
In RL, there is the concept of double criminality which applies to extradition law and (I believe) immigration law as well. In a nutshell, it means that one country will consider someone from another country to be a criminal only if what that person did in the other country was illegal under the laws of both countries. So if a cocaine dealer from the UK goes to the USA, he is a criminal from the perspective of the USA.
In response to @BenOsborne 's question and @KyleStrand 's comment about why Picard did anything at all, there is a military law concept of "Conduct Unbecoming an Officer and a Gentleman" that is often an offense that is punishable but that is less serious than murder. In today's US military, "Conduct Unbecoming" may consist of (among other things)
...action or behavior in an unofficial or private capacity which, in dishonoring or disgracing the officer personally, seriously compromises the person’s standing as an officer; an officer’s conduct need not violate other provisions of the UCMJ or even be otherwise criminal to violate Article 133, UCMJ; the gravamen of the offense is that the officer’s conduct disgraces him personally or brings dishonor to the military profession such as to affect his fitness to command the obedience of his subordinates so as to successfully complete the military mission.
Source: Digest of the United States Court of Appeals for the Armed Forces (emphasis mine)
A revenge killing, even if legal in the jurisdiction in which it was done, could be seen as "dishonorable" to Worf in terms of his Starfleet career, and Picard even says so. The behavior impacts his career and how others perceive him in Starfleet. Here's the conflict of cultures inherent in Worf's position as a Klingon and a Starfleet officer - behavior that is allowed in one context is condemned in the other. Picard detects this and asks Worf how he proposes to handle it.