It wasn't even that she wanted to die, per se. The whole point of her visit was to undermine Ishido by showing the lie of him not keeping hostages, which he was very definitely doing in order to force military alliances. By sacrificing herself (either through seppuku as she was initially prepared to do) or by being killed in the explosion (which is literally part of her speech right before--something like "I do this to protest Lord Ishido's conduct--[BANG]"), she's putting him in the position of being forced to release everyone he's holding and undermine his own position. Ultimately it led to the mother of the heir abandoning Ishido in favor of Toranaga.

I agree it doesn't feel like the most satisfying narrative plot, but it's actually almost straight from the historical record. The woman Mariko is based on (Hokosawa Gracia) similarly sacrificed herself to discredit a similar plan by a Tokugawa rival. When the real lord (Ishida) attempted to take her hostage, along with families of generals loyal to one of Tokugawa's allies, a family retainer killed her, then set the house on fire and killed himself to prevent her becoming a hostage. The outrage was so significant that Ishida was forced to abandon his plans.

Whether the real Mariko's sacrifice was planned to undermine Ishida is not clear, but the overall effect was to help Tokugawa, so in that sense the show is true to both the novel and the real events the novel is based on.