He made this decision to play it safe. His reason for doing so is the same reason why he didn't immediately join in the assault.
Kelvin didn't fight the vampires personally. The first thing he did was to assess the situation.
That was his hob as the leader. He couldn't get carried away, so he made sure that everything was in other before he joined the battle.
Even then, he too used a gun together with the others and mowed down the vampires without standing out.
The soldiers were making a lot of progress with their mission. Most of the credit was due to the special bullets that they were using.
Ordinary bullets won't harm vampires much. At most, the bullet will cause a superficial injury that can be healed in an instant.
No matter the physical damage that a normal bullet does to a vampire, even if that bullet severs the head of the vampire from its body, the injury will still be superficial.
