How can I help you?

Well, in traditional British grammar, the rule is that will should only be used with second and third person pronouns (you; he, she, it, they). With first person pronouns (I and we), the 'correct' verb to talk about the future is shall. This link might help you understand https://www.google.com/imgres?imgurl=http://media.llb.re.s3.amazonaws.com/pub/notes/will_would.jpg&imgrefurl=http://languagelearningbase.com/88531/when-to-use-will-shall-would-and-should-will-have-or-would-have&h=1200&w=1040&tbnid=tXWCtaKCibEd8M:&q=When+to+use+shall+and+will&tbnh=160&tbnw=138&usg=AI4_-kSPX_pc5rDA1Q2GM-teVAt-V991EA&vet=12ahUKEwiA6Yi-1_XfAhVsHzQIHT8ADysQ9QEwAHoECAgQBg..i&docid=zQLx1DM_aN9mNM&sa=X&ved=2ahUKEwiA6Yi-1_XfAhVsHzQIHT8ADysQ9QEwAHoECAgQBg

心 From the king of minions.

The latter is correct in the case. We use present perfect tense to talk about actions started in the past and continuing in the present.(In your case. There are more cases for present perfect usage.) You can study more about present perfect on the following site. https://www.ef.edu/english-resources/english-grammar/present-perfect/

I think there's no grammatical or vocabulary mistake. We often use the word "greatest" as the meaning of "largest" or "biggest". Of course, people often say "largest" in the case, but that sentence is also okay.

Xiao Xiao's answer is correct. We can say that sentence this way. I will kill you to keep my secret forever.

Dana's right. They are different in politeness and kindness. For example: A child and his mom are walking along the street in hot summer. Child: "Mom, can I have an ice-cream?" Mom: "May I ...".(This means "You should say 'May I have an ice-cream'") Child: "May I have an ice-cream?"