Like all literature, it is easy to draw different meanings from the same words, but most of the suicidal undertone is simply in the last stanza:
The woods are lovely, dark, and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.
The narrator has been traveling far to fulfill the promises he made to others. He's looking at the lovely, dark, deep woods and thinking how peaceful they are. The woods are a metaphor for death, which is dark and lovely because you don't have to worry about life's hardships. The miles are not distance, but a metaphor for time. The narrator decides he has to continue on, because he cannot sleep/die before fulfilling his promises to those he made them to.
Frost was an amazing writer whose poems beautifully described rural settings. But I think because of his brilliance, it is likely that his poems had deeper meaning than their face-value imagery. But it's also possible that they were one-dimensional, I just doubt it.
