Lines 22-29 Summary

Get out the microscope, because we’re going through this poem line-by-line.

For by my glee might many men have laughed,
And of my weeping something has been left,
Which must die now. I mean the truth untold,

This second soldier is still going through his laundry list of regrets. He feels that not only did he miss out, but other people missed out in missing him. His happiness could have caused others happiness. Laughter is contagious, after all.
Owen throws in a very subtle twist in lines 23 and 24. The second speaker says that something has been left "which must die now." We're not completely sure what that something is, but he goes on to clarify: "I mean the truth untold." So with this guy's death, so much else has died, too—the pursuit of beauty, the fun and laughter he could have shared with others, and finally a truth that he never got to tell before he died. This just gets sadder and sadder.
What's that truth he never got to tell? Stay tuned…

The pity of war, the pity war distilled.

Drumroll please! The truth the second speaker is talking about is how freaking terrible war is.
First he says it's a pity, then he says that, no, it's way worse—war magnifies (or distills) pity (and sorrow and grieving) to the umpteenth extent.
It's the absolute worst. In case you weren't getting that idea already.

Now men will go content with what we spoiled.
Or, discontent, boil bloody, and be spilled.

In line 26 he explains that because of their war, future generations will have to live with what they've destroyed. War tears apart countries—their land, their economy, and also their morale. People are pretty torn up by war, regardless of whether they win or lose.
Then in line 27 he basically says, if they're not content to go on, they're going to continue to hate each other, and to fight.
There will be no end to the madness.
This guy is feeling a heavy dose of remorse and regret, and he's got plenty of time to stew over it in hell. It's kind of like being grounded. Forever.

They will be swift with swiftness of the tigress,
None will break ranks, though nations trek from progress.

Speaker number two is looking into the crystal ball a bit here. He's predicting what the soldiers of the future will do—they'll continue to fight skillfully and dutifully. But considering he thinks war is the most pitiful thing in the world, that's not necessarily a good thing.
"None will break ranks," means that no one will disobey or desert. Typically that's considered a good thing in war. You want everyone on board, working toward the same task. But here it's a criticism. Speaker number two recognizes that if no one ever goes against the grain, war will never come to an end. Everyone will just keep doing their duty—which is to kill.
At the end of line 29 Owen writes, "though nations trek from progress" which is a pretty harsh critique of war. Most of the time, countries go to war because they think the outcome will be positive. They go to war to make progress for the nation, not to move in the opposite direction.
Owen is throwing some serious hate on the topic of war, which is pretty interesting considering he was a seasoned soldier himself.