Bazarov, the contrast to Nikolai, is not that at all as 'pessimistic' as you think. His downfall was because of his pride, and less about his philosophy. In fact, he doesn't seem to be that much of a nihilist. IF we look at page 166 , we can see that he and Anna had 'set off on a botany expedition' and came back just before dinner, which clearly is a long time. Nature has brought the two characters together, and we can even see that Bazarov's face was even “cheerful and even tender”.
And is it not uncanny that he accepted Anna's offer to teach her Latin names of flowers. This concept clearly seems linked with romanticism to me. Both characters use a scientific interest as an excuse for their mutual feelings.
The fight scene between Pavel and Bazarov also starts in a pleasant setting, witht he 'glorious fresh morning' , very unlikely setting for a conflict and we can see later that they develop something other than hatred of each other when Bazarov kindly tries to heal him.
His manipulation of Fenichka, and the rose - a romantic symbol - shows that Bazarov could clearly be capable of romance and love if he really loved Fenichka, it's only because his pride was hurt by Anna that he takes this chance to control someone.
We can also see that when Bazarov uses nature as an outlet for his emotions of love for Anna, such as going to the forest - you can say a wild version of a garden - or into the hayloft, without being aware that he is acting somewhat romantic.
'before there's a forest there' is what he says to Anna when he was almost unconscious. It could be a clue for his emotions of love there.
Many romantic scenes also took place in the garden, (a natural environment that is tamed and under control by people) Scenes such as Arkady's first confession where he tells Katya he would 'never exchange anyone in the world for her, not even her sister' , and how Arkady was hidden in the 'remotest corner of the garden' where he first discovers whom he truly loves, face slowly brightened, and of course one of the main paragraphs that focus on their relation ship on page 253, where an animated setting of the gently pale gold sunlight, the sparrows the chirping, the sunshine, - these two are the young generation of the novel and we can see that it works out for them just as Fenichka and Nikolai worked out in the end.
Bazarov’s death also is not depicted as pessimistic: the way his death is described as a “sleep, sweet sleep” implicates the peace he achieves through death.
Moreover, if we look at the final part of the novel, we can see how Bazarov's grave is set upon where no 'man molests and nom animals tramples upon, on the birds’ perch on it and sign at dawn'. It's quite melancholic and tranquil, definitely not pessimistic, and the young fir trees besides Bazarov can be seen as a symbol of rebirth. More importantly, the last line tells us however sinful and rebellious the heart hidden in the tomb - Bazarov's, there are 'flowers growing over it peep at us”
This shows hat even Bazarov can seek peace, and that life goes on as the flowers that grow over it tell us about 'everlasting reconciliation' which is obviously a positive force, and therefore more optimistic than pessimistic.
In conclusion, due to the apparent romantic links between the characters as hinted through the use of nature’s beauty, and the fact that the central character (Bazarov’s) ending does not seem to be so fatal – but rather conveying a sense of serenity suggests that the novel overall is more optimistic than pessimistic.
