Background Impulsivity and aggression have been associated with all forms of suicidal behaviour and linked to theories of suicide capability. There is a need to clarify the role of impulsivity and aggression in the progression from suicidal thoughts to suicide attempts and suicide. Method In this naturalistic cross-sectional study, suicide ideators (35), low lethal suicide attempters (37), and high lethal suicide attempters (26) were compared with the Columbia-suicide severity rating scale (C-SSRS), Barratt impulsiveness scale (BIS-11), and the Buss & Perry aggression questionnaire (AQ). Results Physical aggression score (p = 0.032) contributed to the difference between predicted low lethal suicide attempt and predicted high lethal suicide attempt. This model predicting physical aggression showed a fairly weak positive relationship (OR = 1.1) to high lethal attempt and explained 13% of the variance so there is a need for further replications to verify these results. Impulsive behaviour scores in females were significantly higher in the low lethal suicide attempt group compared to suicide ideators (F(2.51) = 3.47, p = 0.039, η²= 0.12). Hostility aggression in females was significantly higher in the high lethal suicide attempters compared to suicide ideators (F(2.52) = 3.53, p = 0.037, η² = 0.12). Physical aggression scores in females were significantly higher in the high lethal attempters compared to suicide ideators (F(2.52) = 6.79, p = 0.002, η²= 0.21). When these analyses were conducted without the participants who died in suicide, men in the high lethal attempt group scored significantly higher than men in the low lethal attempt group (F(2.37) = 3.8, p = 0.031, η² = 0.17), but men did not differ in aggression and impulsivity scores in other comparisons. Conclusion Suicide prevention should address physical aggression, as high levels can be associated with high lethal attempts. Assessment of suicidal patients should address impulsive behaviour with the insight that it can be more prominent in female low lethal suicide attempters. It could be that assessment and treatment of suicidal patients should be tailored differently for men and women. Aggression as a feature of suicide capability could be the link that makes suicide possible.