First of all not all Sikhs hate RSS but yes few do and they have their own reasons for doing so. You don't tolerate someone interfering in your religion and trying to represent your Gurus as Hindus. There is no doubt that Sikhs came from Hindus but you don't need to keep reminding us that we came from Hindus, we already know that. Sikhism is a completely different religion now and we won't accept anybody trying to represent it as a sect of Hinduism. What RSS is doing is highly objectionable and can't be tolerated.
RSS has a it's own Sikh wing called RSS or Rashtriya Sikh Sangat which is headed by a team consisting mostly Hindus. How can Hindus be the board members of a religious Sikh organization which says that its objective is to promote Sikhism? Promoting religion means converting people to your religion so now Hindus will convert Hindus to Sikhism? It is like having a Sikh as the chief of Vishwa Hindu Parishad.
RSS keeps on publishing literature and posters in which they represent Sikhs gurus being blessed by Hindu Gods and showing cow as a sacred animal in Sikhism. RSS has published a book about seven great people, in which they put the name of Guru Nanak Dev Ji, the founder of Sikhism on the same list as Indira Gandhi, the killer of Sikhs. Would they let anybody put the name of Rama or Shiv Ji or Hanuman on a list with any human being or compare Bhagwan Vishnu with Mahmud Ghaznavi?
Why they didn't put the name of any Hindu God on this list of 7 great personalities? Dont they find Hindu gods worthy enough to be included in this list? The reason to put the name of Guru Nanak Dev Ji in this list is to show that he was just a human who can be compared with people like Gandhi and Indira but Hindu Gods can't be compared with Humans.
(Highly Objectionable)
Let's take a look at some more literature which RSS publishes to show Sikhism as a part of Hinduism:
Sikh Gurus never recognized OM.(i don't have objection to putting OM here but this is done with an intention to shown Sikhi as a part of Hinduism).
Cow has no significance in Sikhism. Sikhs don't eat beef but not becauae it is restricted in Sikhism but out of the respect for Hindu sentiments and because of the cultural influence.
Although, Khalistan movement has almost ended in Punjab but Khalistan supporters still exist and all they need is such kind of excuses to start their activities and to gain the support of common Sikhs who otherwise would never support them. But when Sikhs will feel threatened by the activities of organizations like RSS and Shiv Sena, they will be forced by their sense of insecurity to support Sikh hardliners or Khalistanis and let's not forget that Sikhs have for long been deeply wary of the RSS’s schemes and its subversive movement to merge the Sikh identity with Hinduism.