In an interesting analysis of the film (and Luc Besson's work in general) in her book Luc Besson (1998), Susan Hayward explains that when looking at the film's structure overall, the ending is not so much about a character's intentions as what makes sense for the story. Psychologically, when Mathilda is attracted to Leon as lover, she is transferring love for her brother and desire for the love of father she never had to Leon.
Leon plays father, son, and lover. But he assumes that latter role
only when it is too late. Or, put another way, since the social order
of things prohibits him from becoming Mathilda's lover, he gives up
his body for her. He sacrifices the material body for the corporeal
love that cannot be had. Equally significant, he pulls the plug (the
pin of the hand grenade) on his life in exchange for that of
Mathilda's. Having acknowledged his love for Mathilda, the only way
that love can be sublimated is through death, that is, through a
de-phallicising of the masculine body. This notion of the
de-phallicised masculine body is in fact a trope of melodrama. The
phallus is made safe so that the social order of things remains safe
and so that the family does not come under threat...
Besson continues in his description of these two protagonists:
"...Elle lui amene la vie. En acceptant, il accepte sa mort. Mourir
pour donner la vie. Geometrique et cellulaire." (Roughly, She brings
him life. In accepting it, he accepts death. He dies to give life.
Geometric and cellular.)(p. 141-146)
You ask whether Leon intended to die, and we cannot know this this since Leon is only a construct of the storyteller. The storyteller intended that Leon should die and that Leon should accept this death, as repayment for life ("You have given me a taste for life") and love. He also tells her he wants a bed to sleep in and roots, and she gives him that by planting his peace lily. Geometric!
If you try to imagine a world where he survives, where would this relationship go?
Besson did write a sequel to it, by the way, with Mathilda all grown up, but as he is no longer connected to the production company that filmed the original (and has rights to the sequel), it may never get made.
