Well if we go over what we know
Life is ok with her dying but the null specifically can mess up his plans.
Death didnt know the null could suppress there magic and give the host control, so i see no reason life would know about that power as its likely something the past null couldn’t do.
This just leaves the question what is something the null can do that could affect death that life knew about.
Only one idea comes to mind. He could break a spell. Perhaps a artifact or enchantment is present in that room or on death.
Death is known to be reliant on life so she doesn’t die but what if that isnt natural. Life could be placing limiters on death or some other long term control. It is odd that they treat themselves as one entity. Calling themselves the 5th, tow sides of the same coin. You would thing life and death would butt heads, but they dont at all despite their very long history. The other magi have in the past but not life and death. Its odd to get along as equals for that long.