With Radec still in command of the Helghast after the nuke was deployed, Visari wouldn't have been killed if Rico/Sev didn't make it past the colonel, and he likely would have successfully routed and defeated the remaining ISA survivors on Helghan, likely letting Visari gaining better post war concessions during negotiations for ISA POWs.
Templar's actions are essentially the only real reason the ISA managed to fend off the Helghast invasion in the firs game. Without him Hakha would have been captured and tortured by the 3rd army, Rico would have been killed in his suicidal attack on the armored convoy, and Adams betrayal would have gone unnoticed. So without him the Helghast likely would have taken Vekta.
The massive Helghast counterattack in the end of Killzone 2 was pre-planned and likely would still have happened, but with Visari as prisoner instead of dead, the Helghast could have been negotiated to stand down for the sake of their leader. The power vacuum would instead be filled by ISA puppets and likely lead the Helghast down a more stable road, without Orlock and Stahl vying for power
Without the Terracide Helghan would have entered a massive war directly with Earth, and both sides would likely be irreversibly devastated from the fighting and liberal use of WMDs they both stockpile.
And if Kellan lived, he likely would have either become a double agent for both sides or, more likely, him and Echo would have made a more vocal push for peace together.
Or none of that would have happened, who knows? Just my opinions as to the most likely scenarios.