Yeah thats what it suppose to be, sometimes ppl can only brutal kill them when its been a couple of seconds after they were stunned but other times when I just made them into stunned state and I can see the red halo, someone just intantly grapples them and brutal kills them. Sometimes it shows that anyone can grapple them, like one time I just walked past a person stunning an enemy and I was able to see the red halo showing up right when they got stunned and most likely I was able to grapple them.
Thats because during those times while although you may have landed the hit that put them into stunned state, or you ran past a player putting an enemy into a stunned state, no one single player did a high enough % of damage to warrant them being granted the i-frame to claim their kill so the brutal kill is open to everyone since there was not any single person who did enough damage to be able to "claim it as their kill". If SSM made it so whoever gets this hit that puts them into the stunned brutal kill state gets the kill there would be MUCH more "kill stealing". I just think that Assists shoulld actually be counted on the scoreboard and towards your K/D just because there have been numerous time while playing a defensive/team oriented loadout as posiedon i've gone 2-0 sometimes 0-0 but will have gotten the 5+/10+ assist medal.
It seems like with me unless its one on one someone always steals my kill. ugh
like i said if this is happening to you, it's not "your kill" so no one is "stealing" anything. someone either A) Did more damage to the opponent than you did (which would make it their kill) or B) Did an equal amount of damage (there by making the kill available to both of you and you can't "steal" something that doesn't belong to someone else). Try throwing magic out during your cluster fights whenever you have it, it does a good chunk of damage in one blow and usually ensures you get credit for the kills when fighting in clusters.
Now all of this is null if they are just running in and outright killing them or using a WW. But you said brutal kill so I'm going on that information, and there's no way a brutal kill can be stolen unless you were just too slow and the opponent again became available to be killed by other teammates.