Well, there are a couple of different sources of
Voyager comics. There's the Marvel/Paramount Comics series from 1996-98, which ran for 15 monthly issues and a 4-part miniseries that was effectively issues 16-19 in all but name; and there were four different VGR stories from Wildstorm in 2000-01, three graphic novels and a 3-issue miniseries. So that's only 25 comics telling 13 distinct stories.
Quality-wise, they're a mixed bag, but there were some I thought were reasonably good, like Marvel #10, the
Splashdown miniseries, and Wildstorm's
Avalon Rising.
Do they contradict the TV show's better episodes?
Hard to say. There are some minor continuity hiccups here and there, but nothing blatant. Marvel #9 tells a story very similar to the later episode "The Void." #10 involves a time warp to the Battle of Wolf 359 from "The Best of Both Worlds" and shows Klingon ships participating in the battle, which is consistent with a line from BoBW but never reflected anywhere else in canon, so it's unclear whether that fits.
Splashdown shows the Aeroshuttle in use, but we never saw the Aeroshuttle in the show, and I think if it had been functional, there would've been no need to build the
Delta Flyer later on. And Wildstorm's
False Colors is apparently set after "Collective" and is Seven-centric, but there's no mention of the Borg children.
There is a bit of a timing discrepancy in
Splashdown, the sort of error that can creep in when a storyline spans months of real time while taking less than a day of story time. In the first issue, the story is said to take place pretty soon after "Scorpion," since that story was presumed to fall between issues 13 & 14 of the monthly; but in the last issue, there's a reference to the destruction of the Hirogen relay network in "Hunters," which is much later in the fourth season. But either timing reference could be ignored if you wanted.
Did any series characters got killed in them?
Naturally that wouldn't have been allowed of any tie-in while the series was underway.
Do they revisit any of the storylines/places/races that showed up in the series?
Marvel's series features Talaxians and Trabe in #1-3, Kazon in #4-5, and Kazon, Trabe, and Vidiians in #6-8. Wildstorm's
False Colors and
Elite Force both involve the Borg. Wildstorm's
Planet Killer is a sequel to TOS: "The Doomsday Machine."