There's a modern metta gatha which is discussed here and which begins ...
May I be free from enmity and danger
... so I think that's considered normal these days.
I think I read that the rationale is that you should call to mind metta for yourself or your own friends and family -- i.e. people you can easily feel metta towards -- before trying to apply it to people you don't know, and/or to people you feel emnity towards.
Perhaps this strategy is incompatible with trying to practice anatta but I'm not sure that everyone tries to do that anyway -- but see also Metta meditation ever taught to householders?
Also Dhammapada 129:
All tremble at violence; all fear death. Putting oneself in the place of another, one should not kill nor cause another to kill.
That seems to assume that people will inherently refrain from being hurting themselves.
Still I'm not sure metta is really applicable to oneself: partly, because I don't see how that doesn't encourage self-view and clinging; partly, because I suppose that might result in over-indulgence (i.e. accepting your own faults instead of making an effort to correct them), though you might counterargue that true metta would involve properly following the path; and partly, because of think of metta (brahmaviharas generally) as primarily social:
They provide, in fact, the answer to all situations arising from social contact
There's doctrine called "stages of metta".
For example the article -- The Six Stages of Metta-Bhavana -- references the Visuddhimagga (Chapter 9) as its source, and includes ...
- Toward oneself
- Toward our loved ones
- Toward neutral beings
- Toward those with whom we are in conflict
- Toward all sentient beings
... or another source says six stages:
- All four of the above
- The whole unverse
You might see the Visuddhimagga for more detailed advice -- it is quite detailed (15 pages) and begins something like this:
Here is what it says after that, about developing it towards oneself:
(this was quoting The Path of Purification translated by Bhikkhu Nanamoli, from the Buddhist Publication Society).