A ‘brilliant’ scientist travels back in time repeatedly to save his wife from being murdered. His unsuccessful attempts accidentally unravels his ‘wife’s secret life’ and, in turn he ends up killing? her himself. Grief stricken, he must resolve his appalling loop of life, one last time.

(Note: My first attempt for suggestion, correct me if wrong.)

LOG LINE:
With the grounds that the loop is being created with non-theoretical explanations (think: Predestination); one must not state the character’s transformation that the scientist turns into monster in LOG-LINE. It is a journey which audience must come across in the bigger picture. Stating the idea about who the wife’s murderer is, must do. Scientist turning into a monster can be shown as a inner conflict while structuring the script.

ADJECTIVES:
1. For more clear flow of idea ‘Wife’s secret life’ must have an adjective for e.g., instead of ‘secret’ make it treacherous or perfidious or deceitful or dishonest (my preference) whatever the case maybe.
2. Explaining the scientist as brilliant is silly. He is a SCIENTIST, he is supposed to be brilliant. You can try Persistent or Pertinacious or Determined or any other quality that describes your scientist’s character.

Inciting incident, Key Incident and a Dramatic Premise (revolving around stated conflict): This must reflect from a log-line.

Inciting Incident: Wife’s murder.
Key Incident: Scientist’s decision to travel back time and stop the murder.
Dramatic Premise: To Save his wife from being murdered.

Your log line states all this things which reflects a good 120 minutes drama. Good Luck!

APPROACH #1 LOG-LINE:
Writing a log-line and staying in that confined idea is a must. When you write the script you may find shortage of conflicts. Do not engage the script with non-plot conflicts.
APPROACH #2 STORY-LINE OR STEP OUTLINE:
Sometimes log-lines restrict your ideas to certain cubicle thoughts. You do not have space to think bigger. In such difficulties you can always write story or step outline (nearly 20-25 pages) and then, later, catch up with your main idea of the story and convert it into a LOG-LINE. (It is a better approach)