When Ravana was the greatest worshipper of Lord Shiva, why could not he pick up Shiva's bow during mother Sita's svayamwar?
And is Ravana rightly termed to be the cruelest and most wicked rakshasa?
Sita's svayamwar does not happen according to Valmiki Ramayana.
Janaka decided to offer his daughter Sita to a person who can able to strings the bowstring of that bow of Shiva. Many kings failed to achieve this and the name of Ravana is not at all mentioned.
"Hers is a non-uterine birth as she surfaced from the surface of the earth, but fostered as my own soul-born girl and I determined [to giver her in marriage to a bridegroom where his] boldness is the only bounty, [I receive in that marriage...] [1-66-14b, 15a]
"Oh, eminent sage, as my daughter has surfaced from the surface of earth and has come of age, the kings, [having heard my declaration that the bounty for Seetha is boldness alone,] have come and besought for her... [1-66-15b, 16a]
"To all of those kings who are beseeching for the girl, I have not given my daughter, saying that she will be given for a bounty of boldness... [1-6-16b, 17a]
"Then all the kings convoked and on arriving at Mithila, then they wanted to ascertain the calibre [of the bow, vis-a-vis their own...] [1-66-17b, 18a]
For them, those who wanted to ascertain the calibre of the bow, that bow of Shiva is fetched to their proximity, but they are incapable to joggle it, or even to catch hold of it... [1-66-18b, 19a]
While showing it to Rama and Lakshmana, Janaka announces that he will offer her daughter if Rama can able to do it
"Oh, tigerly sage this is that supremely radiant bow, and oh, saint of sacred vows, I will show it, even to Rama and Lakshmana... [1-66-25b, 26a]
"If Rama strings the bowstring of that bow, oh, sage, I will offer my daughter, whose birth is non-uterine, to Dasharatha's Rama..." [So said Janaka to Vishvamitra.] [1-66-26b, c]
Ravana is rightly termed as wicked Rakshasa. An event justifies it is carrying away Sita forcefully by Deception
....And from Janasthana, that mighty Rakshasa monarch, the wicked Ravana, carried away his (Rama's) queen by stratagem and force, deceiving, O sinless one, that foremost of men, through the agency of a Rakshasa, Maricha, who assumed the form of a deer marked with gem-like and golden spots.
Ravana is rightly termed as cruel. Ravishing Ramba is one such example of his cruelty. Along with it, his cruelty can be justified from his following statements to Sita
"O Seetha with best complexion! Whatever limit has been made by me to you, those two months are protectable to me. Thereafter ascend my bed. Above two months you not desiring me as husband will be killed in my kitched for my breakfast."[8,9]