I am astonished that, being a retired IPS officer, you appear to be unaware of the relevant legal position.
Can you point out any law that makes registration mandatory for every organization? Indian law permits many organizations, associations, and groups to function even without formal registration. Registration may provide certain legal and administrative benefits, but the absence of registration does not automatically make an organization illegal.
As for your apprehension that RSS is hiding something or engaging in unlawful activities, India already has an extensive framework of laws and regulations that apply to both registered and unregistered organizations. If any organization violates the law, action can be taken under the existing legal provisions, irrespective of its registration status.
If you believe there is a legal gap that allows unlawful activities to go unchecked, then the appropriate course would be to identify that specific deficiency and advocate for suitable amendments to the law.
However, merely expressing suspicion without pointing to any specific legal violation by RSS does not constitute a substantive argument. In the absence of evidence of wrongdoing or violation of any law, such allegations appear to reflect personal prejudice rather than an objective assessment of facts.