Not the same characters

[BadGamer13 0]

Yesterday I start playing Killzone and found that Sevchenco, Rico, Hammer and Narville isnt there. ¿Does those guys die?

[Enforcer_Yayoi 249]

I think Mercenary is like a backstory to them, explaining how their exploits came to be possible. No idea, they could've died for all I know.

[m4ns4hil 50]

I believe this game takes place just after the events of killzone 1

[Shinoda_2qx 28]

It does take place just after Killzone 1, and parallel to Killzone 2. You get to see the nuke land on Pyrrhus far away in the distance during a later cutscene in the game, which happens during Sev's assault on Visari towards the end of Killzone 2. So to answer your question, Sev and company are still alive! 

[ORGAN1ZAT10N_13 19]

I believe this game takes place just after the events of killzone 1

 

It runs concurrently with KZ1 for the most part. Near the end of KZM, you witness the nuke Visari set off on Helgan in KZ1