The key to this question (which was posed to me by Kelvin Green) is the "with their deaths" aspect of the question. The fact that the characters have been killed off is irrelevant. All you need to be able to write a public domain character is for that character to be IN the public domain, which is the case for all of the Freedom Fighters. The Freedom Fighters were all published by Quality Comics until it went out of business in the 1950s, and eventually sold all its rights to DC Comics. The thing is, during this period in time, Quality allowed the copyright to lapse on their characters. This was not a strange occurance at the time, as very few companies actually bothered to renew their copyrights, as Quality quite reasonably did not feel as though there was anything to be gained by sustaining the copyright on the characters. Comics were a month by month enterprise. In addition, even had they WANTED to renew the copyrights, as the terms expired while the company was out of business, they couldn't ANYways.
So there you have it - you CAN write a comic book featuring the Ray, Phantom Lady, Human Bomb, etc.
The next question is, though, can you title the comic Phantom Lady, Human Bomb, The Ray, etc.? That is an issue for trademark to address. Copyright just dictates whether you can use the characters and/or reprint their stories. Trademark protects consumers by assuring that if they see, say, a comic with Batman on the cover, that the comic book WILL be by DC Comics. This is designed to protect consumers from bootleg (i.e. inferior) material.
The strongest protection regarding trademarks comes when someone registers a trademark with the federal government. However, this is NOT the only way to protect a trademark, it is just the easiest (for if it is registered, the presumption cuts directly to the person/group who registered the trademark). A great deal of comic book characters have NOT been registered as trademarks. Of the Quality Comics characters, DC has only registered Plastic Man and the Blackhawks for trademark protection. For the others, you would have a SHOT, but even though DC did not REGISTER the names as trademarks, the fact that they published a comic book titled The Ray and Black Condor would be a strong argument in DC's favor that they have a trademark on those characters.
So, while you can certainly write a comic featuring the Human Bomb, Phantom Lady, etc., you would probably not be able to advertise it as such.
Their deaths, though, mean basically nothing (except there is less chance for a comic featuring them to be placed into the stream of commerce, which, after awhile, would bolster an outsider's attempts at claiming DC was not protecting their trademark. In addition, if they are dead, perhaps DC would not even TRY to defend their trademark on the characters).