If so, what would be a better way to phrase that sentence? I don't think "spread" is the right word.
Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.
Although there's nothing necessarily wrong with mixed metaphors, this sentence isn't one—and it's fine as it is.
The initial resentment did two things:
- It poisoned the relationship.
- It grew with every perceived slight.
It's not the poison that grew, but the initial resentment. The sentence is a simple conjunction that takes the form the initial resentment did A and B. Both things are natural consequences of resentment.
An actual mixed metaphor (from Merriam-Webster) is:
: a figure of speech combining inconsistent or incongruous metaphors
If we want to get ahead we'll have to iron out the remaining bottlenecks.
Here, the sentence doesn't flow naturally. You have to read it several times in order to try to understand the meaning.
"Grew" seems less mixed than weak. Or overly vague or lazy. Try "killed" "blighted" "liquidated" etc.