Most spirits are probably aware they could kill their hosts the same way they know they can posses things or people and can judge which would be ideal depending on the circumstances.
If Spender straight up told Isaac he can't tell him things because his spirit will kill him it'd mess him up. He's already paranoid over minor things, what child could handle living in fear of just being suddenly killed at any time. He'd either have a breakdown or do whatever KingC says.
Catnine probably already figured out why they hide everything from Isaac, there's still several good reasons he hasn't killed the boy yet, such as the opportunity to fry Spender in this page. Isaac is still useful because he's still close to Spender and the activity club. If he killed Isaac anyone else he possessed would have weather powers, be suspect, and unable to find out anything Isaac couldn't. He would also have the entire consortium hunting a baby killer.