Why does stannous chloride occur despite the octet rule?
According to Theoretical and experimental studies of the electronic structure
of SnCl2 by photoionization with synchrotron radiation AIP Conf. Proc. 258, 60 (1992)

$\ce{SnCl2}$ has a molecular orbital structure as below (HOMO at the top)

Sigma-antibonding: 36% Sn s, 22% Sn p, 41% Cl s

Nonbonding 99% Cl p

Nonbonding 99% Cl p

Nonbonding 7% Sn p , 92% Cl p 

Nonbonding 3% Sn s, 6% Sn p , 90% Cl p 

Sigma bonding 14% Sn p, 83% Cl p

Sigma bonding 48% Sn s, 6% Cl s 45% Cl p

Nonbonding 3% Sn p, 96% Cl s

Nonbonding 5% Sn s, 2% Sn p, 92% Cl s 

So overall it seems:

There is a total bond order of only 1.

Much of the Sn p and some Sn s electron density has been donated to Cl p orbitals.

There is little mixing of Sn s and p orbitals expect in the antibonding orbital.

additional references:

A Symmetry Rule for Predicting Molecular Structures (This reference explains how, based upon second order Jahn Teller effect, the symmetry of molecular orbitals predicts geometry, for example bent vs linear, and says that the symmetry predicts bent for SnCl2) 

and Metal oxidation state effect in photoionization of gas‐phase metal halides (which further discusses the molecular orbitals of SnCl2 discussed above)