Deceased (please note the difference from diseased) sounds very cold to me, although strictly correct.

Victim does not sound right in English either, although the equivalent would be perfectly correct in other languages I know.

Phrases about loss can easily ring hollow too.

A perfectly possible solution is simply to write
We offer our deepest sympathies to family and friends.
If only one person died, then you could possibly write his, her or [name´s] family and friends as appropriate.

If you use the name, take care again - Mr XXX´s family etc. and not just the first name would be my first suggestion, but it is important not to sound over formal or over familiar. If a person was always known, for instance, as Tom or used a nickname, then Thomas or his surname might sound strange... and that would spoil the whole sensitive message once again!

If several people were killed, I might refer to them as ´those who died´ and
We offer our deepest sympathies to their families and friends.

Keeping it short, simple and not over formal is never wrong.

But as others have mentioned, more context (e.g. who are the audience at the formal address?) - would help to get the register right.